To keep my core strong I do 400 push ups and 120 pull ups per week. It is basic, but it works. Plus I can do them when I travel. After stretching and a little bit of light yoga, I do 100 military grade push ups and 30 pull-ups. I started with the goal of 40 push ups a week. At first it was difficult to get through 5 push ups. But now I blow through them. My pecs / boobs start to hurt at about 80 but I work through them. It took me about 3-4 months to get up 400 a week. Not to mention that my upper body, shoulders and abs are looking damn good.
There has been a definite increase in my core strength. Ab / core strength is very important for overall training. I have felt an increase in my core strength, especially running and up hill biking. My posture is better too. Here is a good article on how to do a proper push up. Next month my goal is 500 push-ups per month.
So push on. 
that’s fantastic steve! I’m a core junkie too. but I never got up to 400 pushups a week. Ok, i’ve got a new goal now. rock on.