it really depends on three things:
1. where you're at
2. how much change you want make
3. and most importanly, how hard you're willing to work
you could go from soft to fit in 5wks, but to be honest you probably won't do it on your own.
most people can't do it on their own b/c it involves radical change, and they simply don't want it bad enough.
having someone to push you will make a big difference, so maybe consider hiring a trainer.
if you do, they should be able to give you a solid idea of what you're going to do off the top of their head.
then they should then put together a comprehensive plan for you that will detail what you'll be doing.
and it won't be cheap, expect to meet with your trainer at least 3x/wk.
We don't know what our limits are, so to start something with the idea of being limited actually ends up limiting us.
Ellen Langer