Most of the relevant upper-body muscles are along the back of your body. Sorry, but these mostly aren’t the ooh-and-ah aren’t-I-ripped flex muscles. The most important areas to target are your deltoids, pectoralis majors, triceps, shoulder external rotators, shoulder internal rotators, trapezius, and rhomboids. These muscles help you to maintain an upright, relaxed posture while running, giving you a smoother, more flowing stride.
Same deal as with legs on reps, sets, and what might seem like too-low weights. Aim for two to three sessions per week. The weights are light enough that they shouldn’t interfere with the quality of your runs, so do the work whenever you can find the time.