have you considered the weight vests? a friend of mine (she's tiny - about 5' tall, size zero) was training for a big mountain climb (in Pacific NWest).. she got a small vest, then also a large vest - wore them both and ran a few miles a day..... just thinking maybe you could wear a vest (or two), then if really ambitious backtop w/ weight as third layer.... the vests WILL stay on well, and if do this, won't stress a backback too much.