I think that this is a very interesting topic, too bad I cannot think of a better title to attract people attention and make them read it.
Anyhow, I am a track whore, I would not mind doing a 20miles on a track. Seriously. But recently I have done almost all of my workouts on unpaved roads in a nearby park. Then last week and this week I did one of my interval workouts on a track and one in a park and I had the following revelation.
Shalane Flanagan said before the London marathon that one must be strong to run well on a London course because it has a lot of little uphills, downhills, turns, uneven surfaces, just like a cross country course. And today I realized what she meant when she said that one must be strong.
Running on an uneven surface requires you not only to be fast but to be strong. Strong in a sense that you must have leg and core muscle strength that you do not really need when running on a track. Tracks are smooth and flat, there is nothing to cause you to “break” your stride. But when you are running on trails or similar non-smooth, non-flat surface with little holes, bumps, turns, hills your muscles must work harder to stabilize you and push you off the ground.
A lot of times I land in a small hole or on a little bump that I do not expect and my leg gives out under me (you know what I mean?), I need to engage a lot of different muscles in that leg and core so that my stride does not break and that I push myself off the uneven ground without any time lost. Also when I suddenly have to go up little hill (which does not need to be a real hill, just a few meters of gentle slope are enough to make a difference) I need to engage hip flexors and core to keep my stride and cadence smooth and unchanged.
This is less profound on a road because it is usually smooth without little bumps/holes but roads are rarely track-like flat and therefore you still need to engage all those different muscles that are not really needed on a track.
Therefore I find it very hard to run in a park but I know that it is making me a stronger and ultimately a faster runner.
In addition, I do not think that the benefits are the same when I am running slowly on uneven surfaces. I am still getting stronger but I benefit more when running fast because impact by uneven landing is great and thus the muscles must work harder to keep me going smoothly.
Now I am sure that this lack of strength was one of the reasons why I was such a horrible cross country runner (together with the fact that I found a 5k too long and thus hated CX and did not give a shit and that I never wore contacts so I did not see where I was going).
The workout I did in a park on Friday, preceded by a 80min bike ride (working on that bike-to-run transition): w/up, 1k @ 5k pace (3:56), 20min @ 4:14 per kilometer pace, 1k @ 5k pace (3:58) w/ 2min jog between, c/d. It was not too bad until that last 1k although nothing was easy-peasy. My workouts are definitely getting more challenging as they include more 10k-specific stuff. Phew.