I’ve given serious thought to taking up cycling as a hobby as well as another type of aerobic exercise.
I ran for ten years until I moved to my current home. There aren’t any good places to run close to here, so I’ve switched to using the elliptical trainer at the gym. I usually do 45-60 minutes 6 days a week.
How would I start? I haven’t ridden a bike since college. I wouldn’t even know how to pick one out.
If you don’t know the variety of cycling you want to do, I would suggest you hit up Youtube and watch some videos of the Tour de France, cross country mountain biking, downhill biking, bmx, trials riding, and good old fashion touring trips. Find the type that you like to try out and then go to your local bike shop and be prepared to be sodomized.
Get one of the employees there to size you up and find the right bike for you. The bikes there will have multiple sizes for each model and you can spend as much or as little as you want. For a first timer, try to spend towards the low end of what you see because you will either store the bike in your garage after the first month or you will be wanting a better bike in the next 6 months.
Spend some time on www.bikeforums.net and learn essential knowledge in the beginner’s area.
Yes, definitely give it a whirl. It’s not that expensive to get an entry level bike, if you find out that it’s not your cup of tea then no big deal. You’ll need to decide between mountain biking and road biking, broadly speaking. Do you want to ride a bike on trails in the countryside / wilderness; or ride on the roads - the two disciplines are very different - different bikes, different type of exercise, different social scene etc. A lot will depend on where you live, you can road bike anywhere, but there will be places where there are literally no good trails for mountain biking - especially in a big country like the US.
It’s a fantastic sport - very social and very physically demanding if you want to take it seriously. Its great on the body as well - low incidence of injuries from just pedaling your bike. The very real chance of crashing, though, levels this out some.
With that said, I’m now off to ride my bike 
Yes, everyone should take up cycling. Unless they’re in Bangkok, in which case their ass is grass. But I used to bike everywhere in and around Albuquerque and Honolulu when I lived in those places, and I never missed not having a car. One great thing about biking: No matter how far away you ride, you have to ride the same distance back whether you like it or not, so you’re assured of even more exercise.
This is a good thing?
I would add in mountain biking that not only do you have to ride the same distance back, but it is always uphill in both directions with a head wind. 
Give it a shot, I started with a mountain bike, and then bought a road bike. I enjoy them both for different reasons.
When you go to get a bike, go to a local bike shop, don’t go to a big box store. Bikes at a place like Wal Mart and Target are just shit.
I always took it to be a good thing. Meant I had to do it and couldn’t wimp out. In Albuquerque I just had an old beat-up bike I bought at some weekend market for a few dollars. Had to replace both tires and the seat right off the bat.
But in Honolulu, I got serious. Bought a brand-new Diamond Back from a good bike shop called, interestingly enough, The Bike Shop. Rode it everywhere. At least three times a week, I pedaled up to Round Top, the small mountain overlooking the city. (Went right past Richard Chamberlain’s small mansion.) Now THAT was not uphill coming back down; that was fun. A few months of that, and I entered in the annual Honolulu Advertiser rally and did the full 100 miles. Not a race, you just had to get through it. (I did it in 8 hours and 20 minutes, including an hour’s rest by the sea at the tunaround point.) I really miss biking.
I think you can get the easiest and most consistent cardio from road biking, compared to mountain biking or bmx. With mtb, the up and down transitions are sharper, and you get less opportunity to control your exertion. Road cycling gives you more opportunity to plan a decent route for climbs and lots of flat spots.
I’d see if there were a chance to borrow a bike from someone or acquire a cheap bike, look for C or D group ride with a local club where you can ride without joining, and see how you like it.
Biking requires a lot of specialized equipment and clothing, especially as the winter draws near: extra layers, knee warmers, warmer gloves… be prepared to spend $300-$700 on a decent new road bike at the beginner’s level. $300 would be a great deal on a so-so bike – Fuji makes or made a cheap one with shifters on the down tube. Otherwise, $600-$700 will get you a nice model with shifting built into the brake levers and an 8-speed rear deralleur with 2 or 3 rings (speeds) at the front derailleur.
Definitely. It’s great exercise, a fun way to get around and you may even find yourself biking places instead of driving.
What others have said - go to an actual bike shop (not Wal-Mart, not the generic big box sporting goods stores, etc) and talk to the folks there. They eat and breathe cycling and can help you pick out a good bike.
Personally I like mountain bikes - you can certainly ride the roads (fat tires just have more resistance than those thin road bike tires) and you can also get off the paved path, don’t have as many concerns about hitting sewer grates, hopping curbs, etc.
Some things people often don’t think about:
Get a really good helmet. Try a bunch on at the store, get one that fits right and wear it every single time. You’re going to spend a few hundred bucks on a decent bike, your brain is certainly worth spending $60-100 on.
Comfy bike shorts - you don’t have to spend $100 on 'em, I look for sales at REI or equivalent and get a pair for $30. Treat them well and they will last for years (I’ve got a 10 year old pair of bike shorts that are still in great shape). If the spandex look isn’t your thing you can throw a pair of shorts over them or get the kind that look like hiking shorts but have a “hidden” pad inside. They really keep you comfortable compared to cotton anything.
Safety gear - assume that you are invisible to cars and that drivers are not really paying as much attention as they should. You will be correct in this. For $20 you can get a set of front/rear flashing LED safety lights that make you much more visible when it’s not the middle of the day. Also pick up one of those inexpensive reflective safety vests, or a little shell jacket in Screaming Day-Glo Lime Green.
Learn how to fix a flat tire, because if you don’t you’ll get one when you are nowhere near anything or anybody. A small pump that clips to your frame is under $20, a patch kit is about $3 and a set of tire levers is about $3. It’s really, really simple to do but much easier if you’ve actually practiced it at the bike shop or at home, as opposed to learning on the side of the road.