Upper body strength is great but make sure to also be doing functional things for your lower body like bodyweight squats, lunges and the runner's touch exercise (you can google it).
For upper body, I think the best thing you can do for yourself is to not use machines. You do want to target all the big muscles, but by using freeweights you'll also get all the small stabilizing muscles that will help the most with your running.
I'd say bench press, pullups, shoulder press with free weights, upright rows, bent rows, shoulder flies, tricep kick backs, bicep curls,dips, planks and situps should be what you do.
They are more functional than what you have been doing and will help you gain helpful muscle instead of bulk. Google them if you don't know what they are.
So- you should continue lifting 3 days a week, but seperate it: back and bis and abs one day, shoulders tris and abs another, and then chest, abs and light leg day.
I can't tell you how heay to lift because I don't know how strong you are. Do 3 sets of 8 reps- start out with a lighter weight and increase so that your last set is hard to finish. I'm sure you'll be able to figure that out with a few tries.
Curls aren't bad, they're just not really functional for running. It's not needed but not 'bad'.
I'd say it's hard to overtrain lifting when you're lifting for running. As long as you don't start lifting every other day in the middle of the season you're fine. If, however, you find a soreness start to settle in that doesn't leave for more than 2 days, take some time off to recover.
Good luck :)