I keep starting posts and then deleting them, because I can't come up with a nice linear reasoning to follow.
One of the things runners often do is to train for things, not knowing if they can do them. A typical example is someone who runs a 10K in 42 minutes, then decides they want to run 40 minutes, so they train for that, and, if successful, they decide to run 38 and they train to run 38; this continues until they fail. When they fail, they go looking for a different training plan, assuming something's missing, or they decide to try a different distance. They see somewhere that a 38 minute 10K means they should be a ble to run a 3:00 marathon, so they train to run a 3:00 marathon.
At this point, most of you are saying, "And what exactly is the problem with that?"
The problem is that this sets one up to fail, right from the start. My thinking runs as follows: You run a race and get a time. You know you didn't train or race perfectly to get that time. If you train to do better what you already know you can do, you should improve.
I say: Race first, then train to do that race. It's backward, but it makes sense.
If you're saying "I want to race a marathon, but I know I can't race that far right now," there's a couple of ways around the catch-22 of needing to race to know how to train to do the race. The hard way is to simply run a marathon, no matter how long it takes or how hard it hurts and start from there. I don't recommend that, though there undoubtedly are some for whom that would work.
Instead, there's a little leeway built into training. You can usually run very well over a two-fold range of distances and okay over a 4-fold range, with a precipitous fall in performance beyond that. [And here I have to point out that there are again exceptions. Emil Zatopek ran the 5K, 10K and marathon in the same Olympics and did spectacularly well; he could also have competed in the 1500m, except for scheduling.] Almost anyone reading this could race a 5K today. You'd then train for the 5K, but race a 10K. Then you'd train for the 10K, but race a 1/2-marathon. Then you'd train for the 1/2-marathon, but race a marathon. Each race, you'd expect to fall apart before the end, because you didn't train to race so far, but you'd still be able to finish. Finally, after finishing the marathon, you could begin actual marathon training.
4 days ago