I wouldn't want to add much weight at all, maybe a couple of pounds, as this would have a negative effect on running economy. I would already consider myself much more muscular than most runners. 6'1, 145 lbs isn't exactly tiny in the competitive distance running world. However I do understand where you are coming from. Ideally I would be training to lose a couple of pounds of fat and add a couple of pounds of muscle.
No suggestions on any type of nutrition/workout plan?