My decision "Power or Cardio?"

So I have a problem. I am training for the 10K O.C. Mudd Run July 18th. So I am obviously training by running, but by doing that, I see that I am losing strength during my workouts in the gym with weights. Today I had a hard time benching 135lbs x 15 times when I usually do that for my first set then go up to 185lbs/215/265lbs!!!! So today doing only 135lbs on bench made me feel.....normal. I take pride in my bench press. =(
The problem is that when I lift too heavy, my running gets harder cause I feel too tight and have a hard time running. See when I run, I like to feel loose and relaxed. But being tight from lifting weights ruins my runs I cant stand having pain from sore muscles and have sore legs while running.
But I know that I have to keep training for my runs, but have to find a balance in keeping my strength. I will have to get push ups in after my runs and do pull ups as long as I am in the gym where they have pull ups bars.
So I guess I have to just forget about being the one that puts a lot of weights on the bench press. I have to just run, stay with light weights, kick ass July 18th, and get back to the bench press after the run.......here we go!!!!!