Keep an eye out at kid's consignment stores. I knew we'd probably never cough up the money for a rubber tired stroller either but there was one of these parked outside my favorite used kids' store last fall:http://www.amazon.com/Liberty-Limite.../dp/B001V7QUAU
They only wanted $80 for it, and I had enough credit from taking clothes in to them, that I only had to pay an extra eight bucks. I never packed something into my car so fast, lol!
It's a great stroller, and DS loves the steering wheel on it. Plus it has a couple of speakers towards the top, and a compartment on top where you can plug in an mp3 player and listen to music while you walk. We used to go out some in a regular stroller but the plastic wheels made it so unpleasant on the rough broken sidewalks that we have to navigate. With this thing I can go over all kinds of bumps and holes, through puddles and slush, you name it! I used to be too sedentary too, and now as long as it's over 40 degrees, we get out for at least an hour a day, and I can really get my heart rate up.
If something happened to this one, we would most definitely find a way to purchase a new one outright. Maybe you can ask for money towards an all terrain stroller for Mother's Day, Father's Day, or birthdays?
As far as just walking on foot, we do that too a fair bit, but it is definitely not to give ME exercise. It's great for DS though. He's not much of a stop and look kind of guy, he likes to trot along to get to wherever we're going. So I don't expect him to walk more than maybe 3 blocks, then a rest, then 3 back, tops.