I've done it before, and am about to get back into the program. If you're worried that you can't do the first day, why not start walking 20 minutes every day for a few weeks? That's a good way to ease into it gently.
I really like this program! It worked well for me, and I found that between running 3 times a week, and doing strength training on the days I didn't run, I lost 20 pounds in a few months.