Push ups can be very beneficial; however they need to be combined with other things as well. There is no reason to not do a mini routine and slowly ramp up as you are doing. Push ups, sit ups. I wrestled from age 13-17 and push ups and sit ups were the core of our workouts. Yes, we ran stairs some times, but various types of push ups and sit ups really were all the work outs we did. I could probably knock out 1,000 push ups throughout the course of a practice. As could our coach, a then 62 year old Cash Stone (Look for him in the Matthew Modine Movie "Vision Quest"). Yes, he was 62 and could do everything us kids could do. Watching him knock out 30 hand stand push ups (against the wall) was incredible.
Take a look at the various types of push ups and add a couple of those in and add some sit ups as well. Get that core engaged. And there are more that one type of sit up as well. Sounds like you are doing some decent cardio with your commute, and keep working on the other areas. That's my next goal, hit those other areas as well.