For exercise purposes, I personally would start a lot lower (say 15 lbs.) and work up gradually, adding a pound or two per week. Starting with a really heavy pack can cause stress injuries. As with all exercise, it's better to start slow and work up. Otherwise you might just end up having to stop altogether for a couple of months!

I also wouldn't bother trying to carry more than your eventual pack weight, for the same reason. I have found that climbing hills (or stairs or bleachers if you live in flat country) is far better preparation for backpacking than carrying a heavy pack--even for those where I carried a too-heavy pack.