I'm now on day 35 of the Lean program after my partner in crime bailed and I quit for a month. I thought I was going to die the first two weeks, but I pushed myself to complete all of the daily regimes and hung in there. Week three got easier and I was able to do maximum reps on the most difficult(for me) work outs.
I highly recommend following the meal plan, and as was already mentioned, getting adequate sleep. I had more cravings the first two weeks than I ever had during my pregnancy! I've seen a good payoff, even after my break: muscle definition beginning to show in all the right places, increased stamina, better sleep, more alert, etc...
I didn't do any preparation at all, was completely out of shape to begin with, and probably would've failed the fitness test had I taken it. It's a flexible program, so do what you can. Feops gave good tips. I second pausing the DVD if need be.