Are you doing proper pushups or just wasting your time?
Time and time again I see people wasting energy and time by not doing correct push ups. when done properly Push Ups can be a very useful exercise. I was once asked if i had started bulking up by doing heavy bench as my chest had grown significantly. No, the answer was Push ups.
ensure you have the correct form and you will go far. you don't need to do 100 in a row (although you should check out 100pushups.com to see how you could do so). Just make sure you do them right.