And while you're getting in shape, there's no reason you can't take it slow. Or stop a few times to catch your breath. Or walk the bike up that tough hill. You'll eventually be riding up. You're not standing the whole time.
Cycling is also low-impact, unlike exercises like jogging.
Cycling doesn't stress your knees. Even though my route is entirely flat, I was also nervous about whether I was too flabby to do it. I spent a couple of months improving my aerobic fitness on a stationary bike in a gym before I ventured onto the real thing. You might want to do the same to work up your confidence. Yep, you probably will.