I'm no genius, but I know this, it makes sense to keep your workouts in the same realm as your races. What I mean is, I road race, so I do my mile repeats on the roads. I have hills in there, up and down, there's loose gravel, good footing, bad footing, etc. If I was a track runner, I would do my repeats on the track. If I was a trail runner, I would do repeats on the trail.
You hear runners and coaches talk all the time about knowing the race course, preparing on similar routes, hills, etc. It is important to keep your training like your racing. Before you say, but, but, but, I know a little cross over is good for the body. I know, but primarily, stick with your environment. Train specific.
Today, I ran about a 1/2 mile warm, then ran some mile repeats as follows:
Mile 1- 6:45
Mile 2- 6:30
Mile 3- 6:23
Between each repeat, I rested around 3 minutes. My repeats included significant up and down hill stretches. I then jogged another 2+ miles. 5.5ish total today.
This workout finishes my week of running and totals me out at about 34 miles for the week. This is my strongest, non-race week in a long time. I am tired, but have some significant running on so I am glad to have Sat/Sun off.