What do i do once i reach my target heart rate when running?

Exercise: We generally recommend aerobic exercise to target rate for at least 20 min. Of a 30min. Exercise session. If progression is desired increases in intensity or duration of exercise at two week intervals(small ~10%) are next.

Few options: When you reach your maximal heart rate using target heart rate zones you have many options depending on your goals. I am assuming you are in good shape. Staying there for one or two minute and then slowing down and alternating will increase endurance and cardiac conditioning. Push yourself slowly every time to increase level. Again, I am assuming excellent conditioning and prior doctor check.