Read the hundredpushups plan, as you work up there is a break in between.
Site says: For example: let's say you managed 8 push ups. Looking at the second column, Day 1 begins with Set 1 (6 push ups), a rest period of 60 seconds, before moving on to Set 2 (6 push ups). Rest for 60 seconds and continue with Set 3 (4 push ups) and Set 4 (4 push ups), before finishing with Set 5 and as many consecutive push ups as you can comfortably manage (at least 5, but not so many that you damage muscle tissue). The 60 seconds rest between each level should allow you to complete the workout, but I promise it will get tough towards the end.
Today's blog on my site is about this challenge... It's been harder than I expected. I think I may have re-injured my back (I have some old back injuries) during squats and hopefully I'm not pushing it too much... Maybe I'll have to stick to the push-ups and sit-ups for now.
Yesterday was the start of week 4 for push-ups and squats (week 2 for sit-ups) for me. I felt so weak compared to last week! I took the whole weekend off (except for light exercise in the pool), so maybe that's why? Has this happened to anyone else? Today I did aerobics and hoping I'll feel stronger tomorrow.