Muscular endurance in the upper back and deltoids is key to maintaining good upper-body running posture especially during a distance run like a half or full marathon.
To complete the exercise, stand tall while holding light dumbbells in front of your thighs, palms facing your thighs. Keeping your elbows locked, raise your arms in front of you until they're parallel with the floor. Keeping the dumbbells at this same level and your elbows locked, move your arms laterally until you've formed a T with your body. Next, bring the dumbbells back to the front of your torso, and slowly lower them back to the starting position. That's one rep.
Do 10-12 reps. Increase the intensity by doing 12-15 reps. Standing on a balance element such as a BOSU or balance disc also increases the intensity as well as recruits core and lower-body stabilizer muscles.