You will see the term hill repeats baked into many running plans, because running hills builds stamina and strength. Fighting gravity as you climb the incline requires a serious amount of energy, but think of it as strength training for your legs. If you can't find a hill, don't forget about the treadmill — you can always work the incline to challenge yourself, like with this 40-minute "hilly" treadmill workout.
Image Source: POPSUGAR Photography / Ericka McConnell