From the source:
"The research out of New Zealand rounded up a group of runners to complete a 5K time trial, followed by a six-week hill training regimen. At the conclusion of that training, they challenged them to another 5K-time trial, discovering that their times improved, on average, by an impressive 2 percent. For someone who runs a 28-minute 5K, that's nearly a 1-minute improvement."
More info: http://www.active.com/running/Articles/3-Hill-Workouts-for-a-Faster-5K.htm