Running hill repeats simply refers to running up an incline at a high intensity and running down at a much lower intensity. Like interval training, this type of workout is great for improving your speed, strength and cardiovascular engine. Hill training can be regarded as speed training in disguise, because it is aimed at improving your fitness in the same way as doing Interval training on flat surfaces.
The gradient of the hill, your level of fitness and the intensity of your effort will determine the speed at which you can run uphill but should not be compared to the pace that you can run on the flat (for all obvious reasons).
Hill repeats can vary from short hill sprints at a high intensity, to long hill repeats at a moderately high but sustainable intensity. Short hill sprints (i.e. 1minute intervals) focuses on improving your strength and short distance speed, whereas longer hill repeats (i.e. 2-5minute intervals) focuses on improving stamina and speed-endurance.
Incorporating hill training is essential when you are preparing for a race with hilly terrain, but it can be added to your program even when you are preparing for a flat course. Love them or hate them, but hill workouts can also be done very effectively on a treadmill. Treadmills allow you to choose your desired incline level and can be a lifesaver if you live in a exceptionally flat area where hills are difficult to find.
The same rules that apply to interval training also applies to hill workouts: Ensure that you warm up properly before your workout and be cautious not to sustain injuries by going too hard too soon. Build your hill running capability by starting with shorter sessions at a manageable effort and build up the volume of your workout one week at a time.