I can only comment on my own experience of what cycling has done to me, physically speaking.

Two years ago I weighed somewhere around 280-290 and cycling 5 miles in half an hour left me feeling like I'd done some pretty serious exercise. Two years and about 3000 miles later I still weigh somewhere around 260 but I've lost around four inches from my waistline and recently cycled 33 miles in a little over two hours on a mountain bike (I mention the mountain bike because a road bike would have been quicker).

I still eat much the same kind of stuff as I always did although less of it and I try to eat less junk than I did before. Cycling means I can burn off calories which trims my waistline, although at the same time it means my leg muscles are visibly larger and stronger than two years ago. Muscle is heavy, so despite being vastly fitter than I was two years ago (and probably fitter than I've ever been in my life) I'm still very much a Clyde.

If I cut some of the stuff out of my diet completely I don't doubt I could drop below 200. A good friend of mine is very much fitter than I am, he's not much shorter than me and I think he's somewhere around 180-190. But then he spends hours on an exercise bike, he rides to work every day, he runs half-marathons, and I'm just not that heavily into it all. I do like the idea of dropping under 200, but then I like cake too.


ETA: Whatever other people around you are doing the best thing to do is start where you are. If you can cycle 50 miles in a single ride aim for 4-500 miles per month. If you can cycle 5 miles and then feel ready to collapse aim for 50 miles per month and slowly build up your endurance. Whatever you're doing, if someone else rides 5 or 500 miles in a day doesn't really matter - too much comparison leaves you either smug and complacent or horribly disheartened, neither of which helps you improve.