Two posts ago, I advocated training for the race one just did - putting the cart before the horse, as it were. This gives one an obvious goal time, if one's racing the exact same race, but that never happens. One of the things I needed to mention was how to set a goal for a different race.
If one's just changing distances, there are a lot of ways to come up with comparable times. A common and simple one is the Mcmillan calculator, based on the work of Jack Daniels (and McMillan, of course); this seems to work best for runners that do best at the marathon distance. An older method, by Gardner and Purdy, assumes one to be equally good at all distances and is more reliable for those specializing at shorter distances; it and other methods are compared here.
If one's not running on roads or tracks, but on trails, things become more difficult. The best easy method of comparing times is to rely on what others have done, such as what one finds for ultramarathon distances at realendurance. Comparison of hilly races or winter races is even more difficult at shorter distances. There, it's best to look at your past race results, find who finished near you and then look up their results - assuming you can find anyone who's run the race your planning on doing. Locally, Apple Raceberry Jam works well; if looking at races outside Minnesota, athlinks is probably the best source.
Using other runner's results to predict your own is risky, as different runners are better at different distances. Other surprises can occur because some runners train through early races and only peak for certain ones. For example, one year I raced against the same guy repeatedly; I'd start ahead of him, he'd catch me and then I'd pass him again before the finish. This happened almost weekly, so I started to use him to gauge whether my own pace was too fast or slow. Then, in one race, I saw him take off ahead of me early, so I figured I must be going too slowly and sped way up to catch him and I quickly fell apart - only later did I find out he had "just jogged through" all the races except that one.
Lost in Space
2 days ago