Pushups, hiking and pullups

Started todays workout with a closegrip pushup ladder - made it to the 10th minute and did 6 reps on the 11th minute which I think is a PR but not sure.  Immediately following I hiked the ridge at Runyon which took about 30 minutes.  When I got home I continued with the fighter-pullup-program.  Today it was 5,4,4,3,3 reps.  I've been noticing that my midsection has been sore on and off the past couple of weeks and I couldnt figure out why.  Initally I thought it was something I ate or a possible strain from doing getups.  After doing pullups today I realized that it's the pullups making my midsection sore.  I'm doing strict pullups and keeping the lower body stationary puts tremendous pressure on the midsection.  Pullups give you some serious bang for your buck :)