Hill repeats are really speed work in disguise. Hills are a great way to build speed and power on tired legs. The power required to run up a hill is the same as to run fast, but the leg speed can be lower. A grade of about 3-5% is ideal, but you can play with speed and gradient to keep the workout interesting. Longer hills like this challenge your ability to maintain power and - especially form - as you get tired. In the second half of each climb, make sure you keep your shoulders back and your chin up.