Once you know what you want to do, you have to decide what you need to do to reach your goal. That depends a bit upon what you've been able to do already.
If you want to finish the Sawtooth 100 Mile - which is on a lot of people's minds this week - what you need to do depends upon how fast you are at shorter distances. If you can run a marathon in under 3 hours, you can finish the 100 and how you train doesn't affect your chances of finishing (it does affect the projected finishing time; how they run the race is the most important factor for these runners' odds of finishing). If you can run a marathon under 4 hours, you can probably finish the 100 with only a few key training workouts [specifically: 1) finishing the Voyageur 50 Mile or a very similar race, 2) running 50K with 10000 feet of climb - which for most means hill repeats, 3) running technical trails at night, and 4) 24-36 hours without sitting and eating only what you expect to eat in the race; for most people this means a 24 hour race]. If you can finish a marathon under 5 hours, you need to train specifically and diligently for months to have a chance of finishing [as I learned, "do whatever you have to do to go into the race confident you'll finish and go into the race determined to finish, no matter what happens"] If it takes you more than 6 hours to run a marathon, you probably won't get past the half-way mark of the 100.
The same sort of thing holds true at shorter distances as well. Though no one reading this will run a sub-4 minute mile, I happen to have the standards on hand for doing it. If you can run 400 meters in 47 seconds, you can run a sub-4 without training for it. If it takes 50 seconds to do 400m, then a sub-4 requires about a dozen specific workouts. 53 seconds means about a year of hard, specific training and even then it's iffy. 56 seconds or more and it's improbable that any amount of training would get you to your goal.
So... I know that I set my goals too high. My motto's been: aim too high, fall a bit short and end up beyond all those who goals were set too low. That worked for me once upon a time, but not now. Now I need to be happy if I improve just a bit, here and there, now and then.
Going up the country
1 day ago