I am 5'0" and 120 lbs and can fit in a fair number of the non-harness booster seats (the ones that use the vehicle seat belt) and just about every jogging stroller made.
FootiePJ, stop worrying and go get in one of the 75lb rated strollers. Now. You won't break it. I've put at least 50 miles on my BOB stroller myself over the last year, and I've also pushed friends around in it, the largest weighing 180. My roommate also used it constantly to transport his 160lb girlfriend after surgery left her unable to walk long distances.