This is subtle reasoning on an esoteric topic, but if you hang in there, I think there's a valuable lesson.
When I started running ultras, my training evolved into something like this: Saturday I'd run at a brisk pace until I involuntarily slowed, then crank out slow miles, so I could do that in the late stages of a race; total would be about 35 miles in 5 or so hours. Sunday and Monday were (1.5-2 hour) runs on rolling hills and trails, so I could get in decent mileage on terrain similar to where I raced. Tuesday would be a hill workout, where I would force myself into oxygen debt, then continue running the downhills after being forced to walk the uphills, because I have trouble running downhill when tired and need to do that in races. Wednesday through Friday were easy runs of 20-30 minutes, allowing for a full recovery.
I think that's an excellent way to train for a 50K or a fast (sub-7) 50 mile, but it completely fails for running 100. It has all the right elements, but they lead to the complete opposite of what I need for the longer race. I was teaching myself to go out too fast on the long run and even the rest days made it easier to push too hard on the long run.
One of the common ideas for beginners doing 100's is to run fairly long on both Saturday and Sunday. I thought for a while that this was simply because this was when most people have the time to train. I only knew one person who ever ran long on consecutive days and I don't think he made a habit of it, so I didn't think it was valuable. What I see now is: frequency = honesty. I could convince myself that my long runs were done easily, even with the fast bits, but I was pushing too hard and only after several weeks would I find that I could no longer maintain my pace. By running long on Sunday as well as Saturday, I would be forced to run easily on both days and thus get a better feel for pace and effort.
To run 100 miles in 24 hours, an "easy" long run would be 30 miles in 7 hours, which is roughly the same pace as the race itself. Running faster is not better. One can figure out from what time one can race the distance what one's long run should be (7 hours at pace works pretty well at all ability levels), but the converse is not true: one can't say from one's long runs what time one can for 100 miles, because it's too difficult to gauge just how hard a long run is to do. They all feel equally hard to me!
Why am I alive with a long road ahead!
1 day ago