There are people meant to be good runners. They people can do an intense strength training session and no gain muscle mass no matter what they do.
If you're gaining mass on 2 sets of 5 reps without consuming mass quantities of calories then you probably shouldn't be running.
I meant to say HIGH intensity, HIGH volume work is what leads to optimal gains in muscle mass 3-5 sets, 8-12 reps, that sort of stuff. I just mistyped.
50 pushups, 1000 situps, 200 pullups and other such lunacy that most runners do does nothing more than lead to fatigue. Then the runner gets discouraged and stops strength training all today. Those high volume routines, low weight routines DO NOT increase strength.
You CAN increase strength without increasining size. This is what you want. SRENGTH without SIZE. The key is that it is a balancing act with your other training. If you're putting in 100-150 miles a week it's doubtful you have much in the way of energy to lift. So, just 30 minutes a week. If you're a regular letsrunner putting in 50-70 miles a week you could easily hit the gym twice a week for 30-45 minutes. The gains in strength will translate to you being a faster runner AND the gains in testosterone production will translate into better recovery.
Alan