I have the same rack/bag you do, run 25c gatorskins, weigh 185lbs and commute 36 miles RT. My last rear tire lasted for over 4000 miles without a flat and I only replaced it when I ran over some metal and put a cut into the sidewall (the tube bulged out a bit but did not flat). Probably could have gotten another 1000 miles out of that tire...

You'll be fine.