I noticed that most of the 100 push ups a day people are NOT former military and there is a reason. The most common surgery for soldiers at about 5-15 years of service is rotator cuff surgery.

You want conditioning from push ups do one that takes you 1 full minute to complete. Of do as many as you can with your lungs empty and do those slow as though you are doing heavy lifting on the bench, slow up and down. Say 10 seconds down and up each. 5 Would take 50 minutes and you ain't starting out that high with empty lungs.

100 a day is a recipe for repetitive motion injuries. People get carpal tunnel from using a mouse or splicing telecom cables, think about it.

I know this may attract hate but talk to any guy that spent time in the Army or Corps and just ask if they knew any guys that had shoulder surgery and why they had it. I am not saying to not work out for the slow kids. I am saying most fitness fads are generally not well thought out.