I've found this good website on how to achieve 100 push ups challenge in just 6 weeks time. It's a very good programme and easy to follow. 3 times a week (30 minutes per day with rest day in between).
I used to be able to do 30 to 40 push ups non stop but now only 20 push ups. The 100 push ups challenge doesn't necessarily need you to do 100 push ups non stop. You can do 10 push ups, rest for 1 minutes and continue until you reach the magic number 100.
The benefits of push up - great for your chest, tremendous job of defining your abs (4/6 packs), triceps, shoulders and torso.
I'll dare you to take the challenge