My trainers would run 20 minutes or more on a tank of fuel but I'm not able to concentrate on flying for that long. After 10-12 minutes, my neck hurts from looking up, and I start feeling pretty uncomfortable overall.

Most of the people in my club set a timer for 10 minutes and land pretty soon after it beeps.

Even though your engine is running well, if your run time is that much shorter than your instructor's, it is running richer than his.