We tend to think of hard workouts in terms of repeats of a 400-meter track. Certainly tracks provide objective data on how fast you run for how far, and that data is easy to compare to how you did last week or last month for the same workout.
But doing the bulk of your hard workouts on a track can be a crutch. It’s easy to get used to getting split times every 400 meters, or even 200 meters, and then adjusting the pace if necessary. But most of us run most of our races on the roads. When was the last time you got 400-meter splits in a road race? Doing too many workouts on the track and judging your success by how well you hit your splits means you don’t develop as good a sense of judging your effort using your body’s feedback. Yet your body’s feedback is mostly what you’re going to rely on in your races: Am I going out too fast? Starting too slow? Can I push it a bit more on this hill? Can I hold this effort to the finish?
A good compromise if you’re starting a training cycle is to do your first few hard workouts on a track to get a sense of what your basic paces feel like. Then do most of your hard workouts by time away from the track; for example, instead of six 800-meter repeats with a one-lap recovery jog, do six hard 3-minute runs at 5K race effort with a 2-minute recovery jog. You can easily translate all the main types of hard workouts into minutes instead of distance.