That all depends on how hard you ride. I’ve heard people say they don’t get fit riding and that it is too easy. It’s only easy because they are plodding along at a low speed. If you put the same effort in so that your heart rate gets as high as it does when you run then you can get the same cardio fitness improvement in the same time. An hour of interval training on a bike is great for your fitness. I definitely agree that it does nothing for your running muscles though. Cycling and running use different muscles and muscle fitness doesn’t translate well from one to the other.
Advice: Have a good hard think about how you are going to ride and get a bike that suits. If you won’t be doing any trails at all you might like a dedicated road bike, you will go faster which means you can cover a greater distance in a given time and see more scenery. If you will doing a little bit of dirt riding but mainly flat non-technical stuff and you will be predominantly on the road, a hybrid might be the way to go. It’s bit heavier than a road bike but with a less aggressive riding position. I made the mistake a long time ago of getting a hybrid when my riding habits really dictated I should have got a road bike. I ended up buying a road bike a few years later so getting one to start with would’ve saved me money.
Whatever you get, no bike is “easier” than another, they just go different speeds for a given effort level, therefore you can get the same fitness from riding anything as long as you put the same effort in. The ultimate truth for all exercise is that it never gets easier, you just get faster.
You can pay a lot of money for minor increments in bike technology. The law of diminishing returns applies. You will get the best value from mid range stuff. It will be well built, ride/shift smoothly, and last a long time without breaking the bank.
If you find yourself turning into a weight fanatic (paying significant money to lose a few hundred grams from your bike), just remember that you can get the same weight reduction by eating a little less for a week ;).
Find a really good bike shop. You will recognise this because there will be a good variety of bikes from cheap to expensive and from mountain to road. The staff won’t talk down to you because you don’t shave your legs, but they will know what they are talking about. This can be difficult to find but once you find one, stick with them, and support them as well as you can because you will need their expertise from time to time. I buy some bike stuff from cheap online stores that bricks and mortar stores just can’t compete with, but I make sure to make regular significant purchases from my bike shop because I need them to stay in business. In return they give me a good discount and good service.
Whatever you do, don’t buy a bike from a department store.
Agree with the advice to use a high cadence (>80 rpm). however using a low cadence at times can be good for strength training but it is hard on your knees so do it with caution. When climbing hills, sit down and spin primarily, but you might like to standup with a lower cadence every now and then to use different muscle groups. If you do this, click up a couple of gears to make up for the lower cadence you get when standing.
Get clipless pedals and shoes, they are great for increasing your efficiency. This applies whether you get a mountain bike, hybrid, or road bike. I would suggest that over anything else that serious cyclists do (eg wearing lycra, drinking lots of coffee, shaving your legs.)
If you aren’t into the lycra look you can get “shy” shorts that look like normal shorts externally but have lycra and padding on the inside.
If you get padded shorts don’t wear underwear with them. The primary purpose of the padding is to wick sweat away and help prevent saddle sores, underwear interferes with this.