Now (indulge me a moment), I've been wanting to get myself in better shape ahead of a rather significant birthday milestone, but riding my bike hasn't necessarily been a part of that, it's just been my way of getting around. However, I made the happy discovery the other day that I've lost seven pounds. I've never needed to lose weight, really, and I think seven pounds is about all I would like to lose (in fact, I'd like to gain a couple back), but I attribute it all to my riding, even just the relatively small amount I do each week.
The point (which isn't just pure narcissism, I swear), is that the health benefits of riding a bicycle accrue even if you're only huffing around on an old three-speed bike a few miles a week. Indeed, pedaling forty pounds of British steel (or American, Canadian, etc.) around is a sure way to firm up those thighs. So, don't let anyone tell you that you need a fancy new bike to get the exercise benefits of riding a bicycle, or that you have to launch yourself on epic treks in stretchy shorts. Any amount of riding, on any bicycle, in any clothing will make you healthier. Ride more, feel better, get healthy, ride more. It's a sweet cycle.
SOURCE: old Sturmey-Archer advert here.