In college, I bought a cheap ($150) mountain bike to use to get to class in Boston. It worked well enough, but was rather heavy and slow. I couldn’t imagine riding it any long distances.
Now that I live out in the beautiful mountains, I am thinking about taking up bicycling, but only if I can go fast enough for it to be fun.
How much different will a decent ($800 or so) road bike be to ride, compared to what I’m used to?
Well, it will be lighter and the rolling resistance will be lower due to higher-pressure tires that have a smaller contact patch. That said, the advantages the new bike gives you will probably (initially, at least) be overwhelmed by years of not riding and by increased elevation and gradient. Still, if you persist, you will get past that “OMG, Why did I think this was a good idea?” phase.
A more expensive mountain bike will be lighter and faster than your old one. A road bike will be even faster though I’m not sure by how much.
I did finish several metric centuries on a mountain bike so long distance is not out of the question.
It makes a lot of difference, although the biggest bit IMO is the tires. If you put narrow high pressure slicks on a lighter, hard suspension MTB you will get about 1/2-2/3 of the advantage of a road bike. The rest is weight, fit, riding posture, and 700C wheels having a bit of an edge over 26"ers…and you can run really high pressure tires.
Getting the right fit on a road bike is critical. If you want to ride far and fast, you need to stay down on the grips, and you will suffer if the fit is poor.
I have a heavy cheap mountain bike, and I bought (last year) a used TREK hybrid. I love it. The difference is night and day, really.
Oh, another thing is that a road bike will be geared taller than a MTB. This gives you more gear choices at higher speed, but you lose the stump pulling low gears most MTBs have.
Since you hope to ride in the mountains, make sure you get a bike with a triple front chain ring…This will give you some granny gears for climbing while you work on your conditioning. Triples are fairly normal on recreational/fitness targeted bikes, but a lot of racers want to save a few grams if they don’t need the extra gearing range.
More on fit: If the bike doesn’t fit, you will never be comfortable, but even if it does, you need some conditioning…back, butt, neck, etc. will all complain at first.
A good bike shop will offer fitting service, to help you choose the right bike. Done right, this is worthwhile, but there will still be some trial and error to find what works best for you. If you get a bike with an appropriate frame size for your build, you can tweak seat and bar position to get it exactly right once you get past the newb aches and pains.
I had a good quality Cannondale road bike, and a good quality Trek mountain bike, and that road bike was VASTLY more efficient. For me, by a factor of 6 or 7. I used to routinely do 60-70 mile rides on the road bike, and I struggled to do 10 or 12 on the mountain bike. Very heavy rims and spokes, and especially fat knobby tires, made a BIG difference.
There are lots more options for mountain bike tires now, including ones that have solid rubber (no tread) in the center. If you’re riding on paved roads, knobby tires slow you down a lot for almost no benefit. Look at new tires before you look at an expensive new frame.
I agree with this. If you put narrower rims and tires on a road bike, let alone a mountain bike, it will make a noticeable difference.
When I was just out of high school, nearly 30 years ago, I went to work for a friend who owned a bike shop in exchange for parts. I built what was essentially a $3000 custom tour-racing bike. You could pick it up with your pinky finger. I could easily outrun city traffic on it. I put some real granny gears on it and there were almost no hills I couldn’t go up. No mountain bike can do that.
Given improved technology, you could probably get something nearly that good these days for about $1000, even in spite of inflation. If you are paying that much, your bike shop will be happy to do custom gearing for you.
Don’t underestimate modern top-end mountain bikes. Cannondale’s Flash Ultimate is around 17 pounds and is as fast as lightning.
It’s also $9500.
I never liked mountain bikes, because all I ever do is ride on roads, so I haven’t seen any new top-of-the line ones recently. That said, if all you are doing is riding on the road, a mountain bike is a poor choice. It has features and such that are optimized for off-road use. Those range from useless to negative in on-road situations. For the same price, a road bike is better for road use than any mountain bike ever will be.
On the other hand, I would never take a road bike off-road, ever. If you want to do both with the same bike, it would have to be a mountain bike like you describe, because while a mountain bike will waste some effort on-road, a road bike will get stuck or break off-road. Your suggestion is a better “does both” compromise, if compromise is what you want.
My advice is a little old, as I haven’t ridden much in 10 years. However…
The difference between road and mountain bikes is huge. The tires are a big part, but the posture is too. Road handlebars are much better for the road, with at lest 5 distinctly different possible positions. Wind resistance is much improved. I think the posture on a mountain bike is partly dictated by the need to jump and twist and handle huge bumps. Crossing a creek full of slimy babyheads is a challenge with mountain bike posture, but completely unthinkable with road bike posture. But the road posture is way better for maintaining a smooth line and graceful curves and sticking to a narrow track. It is pretty practical to keep your tires on a single painted white line. The frame wobbles back and forth less on a road bike when you are working hard, and you spend more time in the saddle.
One thought about road gears - for the mountains I would want a low, low granny gear, and I would want nice even steps up from there, not just one super low gears and the rest close together. Racers who ride fast have much more wind resistance; the work they do is proportional to their speed cubed, I think. Therefore there is not much reason to have very low gears, as the reduced resistance at climbing speed takes away most of the normal burden and these guys have plenty left for gaining altitude. For most of us, though, hill climbing is too big a load without lower gearing. Therefore, while the fastest bikes for the fastest riders have narrow gearing, the fastest bike for me and probably you would have very wide gearing.
One other thought - I always found road pedals were a hassle to get clipped into, because they only have clips on one side and it is clumsy trying to turn them right side up. You don’t need this while trying to start across a busy intersection. If I had it to do over, I would put mountain pedals on my road bike too, so I can clip into either side without looking down.
When mountain biking, you kind of expect to take a few spills, and even without falling you expect to get scratches. As a rule, mountain bike spills happen at less than 5 or 10 mph, and you remount and keep going, or go back to do it again to master a skill. Not so with road biking. A spill from a road bike could be at 20 or 30 mph, even 50 going down a steep hill. It is often a career changing experience. Pavement is so much harder and so much less lubricated than grass or mud or deep water. There are very few thorn bushes on the road, and very many cars. So road biking isn’t about handling spills well, it’s about not having them. I think I took an average of one spill per hour mountain biking, and ten scratches per ride, but while road cycling a few thousand miles I only had perhaps 3 or 4 awkward dismounts and never a spill proper. Fortunately. When you come home from mountain biking you should be muddy and a little bloody, but from road biking you should only have bugs on your front.
Tailwinds!
Been there done that. At first I had a hardtail Cannondale that I put a set of Tom Slick narrow high pressure tires on. It was much better than the knobby tires but my riding buddy wanted to do road as well as mountain so I bought a real road bike.
OMG! the difference was unbelieveable. With the mountain bike acceleration was a chore, with the road bike it just moves faster and faster.
If hills are an issue, get a tripple chain ring in the front.
For me at least it was a night and day difference. For someone coming off a $150 mountain bike it will feel like being on a motorcycle IMHO.
Kevbo basically has it.
90% of the work you put into riding the bike is going to be associated with the wheels.
Knobby tires force you to do a ridiculous amount of work that’s completely needless when you’re on-road.
The weight of the sturdy rims on a mountain bike themselves is also a substantial contributor to the difficulty of pushing the bike down the road.
The actual weight of the mountain bike’s frame isn’t what’s screwing you up so much, although it doesn’t help things.
Even the cheapest road bike you can find in a real bike shop will be a massive improvement over a mountain bike that has tires suitable for serious off-road usage. If you know your size, it might be a good idea to try your local Craigslist for a good deal on a late-model bike whose owner broke his new year’s resolution and hasn’t been riding it for a year.
In Hawaii, I had a fairly pricey Diamondback mountain bike that handled very well on the road. I rode it all over the city and never felt it was difficult. Even rode it 100 miles in the Honolulu Advertiser newspaper’s annual bike run. (I was in much better shape back then.)
I had an $80 heavy (all steel) mountain bike bought second hand which I used to try and do long rides with.
Then I bought a $1000 giant CRX2 flat bar road bike… I can ride more than twice as far and feel like I have made the same or less effort.
Whether you think that’s worth it or not will depend on how much you earn I guess. 
Roughly speaking, air resistance is proportional to speed squared, rolling resistance (from the tires deforming) is proportional to speed, and bearing friction is independent of speed. The weight of the bike will be relevant to bearing friction (which should be very small, anyway) and to rolling resistance, but remember that it’s your weight plus the weight of the bike that matters, so losing ten pounds on you is just as significant as losing it on the bike (even aside from the fact that losing ten pounds probably means building up muscle, too). Rolling resistance is reduced by increasing tire pressure and by making the tire smoother, and air resistance is reduced primarily by reducing your cross-section (mostly done by posture, leaning over more). Weight is also relevant for acceleration, and rotating weight (i.e., in the rims of the wheels) is more relevant than weight in you or in the frame for this purpose (it counts for about double).
Enough with the theory, though, from my personal experience the biggest difference between a road bike and a mountain bike is the shocks. Shocks are great if you’re going over rough terrain, but on a smooth road, they’ll just provide one more place for energy to be dissipated. I’m used to a hard frame, and when I try to ride a bike with shocks, it feels like I’m riding on marshmallows instead of pavement. There are some high-end bikes with adjustable shocks, where you can flip a switch and get the benefit of shocks or hard frame, but if you’re in the market for something like that, it might be better to just get two different bikes, one for road use and one for off-road.
For some reason this thread stays on my mind.
Price: The $800 figure you mentioned is realistic based on your stated needs, assuming you can buy clothes that fit you at Walmart or Target… meaning you are not exceptionally tall or short, and need a custom frame. It won’t be anything a racer would ride, but it will be reliable, fit, and make you feel fast until (if ever) you get in racing condition.
If you wait for last year’s model on closeout, you could either spend a couple hundred less, or get something a bit nicer. If you know what size frame you need, and know pearls from swine, you could pick something nice up on craigslist or ebay for about half that…since you asked here though, I;d suggest new unless you can locate an expert to help you shop.
Just remember that bicycles are sold at shops that have a full time bike mechanic. Big box stores sell bicycle shaped objects that will reveal their true nature in six months or 500 miles, whichever comes first.