Exercise!

I recently found thanks to Lifehacker, hundredpushups.com. This involves doing a fitness test on the number of pressups you can do at point 0 (now), and then suggests a program to build this up to 100 pressups over a number of weeks.

I have recently been using the gym and running a lot, as preparation for my next half marathon in Bristol.

My question is - for anyone out there with half a clue about these things, how should I combine these things? Currently I alternate running, and gym work daily. Should I try doing these pressups on a running day, or on a gym work day?