I am thinking about following one of Hal Higdon's programs, which includes hills. Unfortunately, Hal's definition of "hill" is very vague. Because there are no hills where I currently live, I am using a treadmill. I can set up to 15 degrees of incline. Currently I am running several repeats of the following: running up a 9 degree incline for 4-6 minutes, followed by 3-4 minutes of running at the same speed at zero incline. Does this make sense?
The word "hill" does tend to be vague simply because it depends on nature to create those hills, usually. Like running outside, they will be inconsistent from one hill to the next. According to this article, a hill can be mimicked on a treadmill at between 4-7%.