In reply to Tyler: I generally do hill reps rather than intervals, so don't mind if it takes a while to descend.
Unless you're after particularly long hills, are there any hills nearby that are sufficiently long that you could halt the interval part way up, and so don't have to return to the beginning to start the next faster interval. So if you can find a hill that takes about 10mins to climb, push for 2 mins, then easy for 1min, and repeat. At the top walk down and repeat the set.
For anything longer I'd just plot a hilly route and push on the uphills.