For most runners, this is a short run on grass or dirt, rather than road or track, preferably with some hills. Runners tend to lose some agility and balance if they only run on smooth surfaces. Running on hills, and running on uneven ground, use the muscles you use in running in a slightly different way. Cross-country and trail runners usually combine this with other workouts and I'll cover that soon in a different post.
If you want to be really detailed about it, these runs should feature hills the opposite of what you do in your interval runs: if you're doing 3-5 minute intervals that week, you should run hills that take 0.5-2 minutes; if you're running 0.5-2 minute intervals, you should run hills that take 3-5 minutes.
Long hills should be 4-6 percent grade, short hills 6-8 percent. Finding appropriate hills can be challenging, so many runners resort to inclined treadmills, but that eliminates the terrain aspect.
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