Once you've decided what race you want to do and what time you expect to run, you try to divide it into parts and train specifically for each part. It took a while for me to learn what that meant for 100 mile trail races, as they didn't fit nicely into my ideas for shorter races. Each coach has his own way of dividing things, which forms a sort of philosophy of training. Once you start comparing them, it's easy to get lost in terms: aerobic capacity, anaerobic capacity, anaerobic threshold, lactate threshold, lactate tolerance, stamina, endurance, strength, speed-strength, specific endurance, sprints, strides, speed skills, extensive intervals, intensive intervals, pace intervals, specific endurance intervals, repetitions, muscular endurance, time trials, fartlek, ladders, pyramids, cut-downs...
I divide things functionally, by breathing patterns and stride rate. Here's the types, separated by letters just to give them names. U: When walking, recovering, or at ultramarathon paces, your breathing pattern will be irregular and you won't be thinking about it (right now, if you try to measure how many breaths you take in a minute, you will change how you breathe - you can't help it). A: Somewhat faster, you'll start breathing at a slow regular rhythm that matches your stride, generally breathing in for two steps and then out for two; again, you probably won't be thinking about it and you will occasionally break that pattern (like when going uphill). N: Faster still, you need to breathe more often and you switch to breathing in when one foot hits the ground and out when the other one does. M: If you run even faster, you can't change that breathing rhythm, so you start taking faster (and sometimes shorter) strides. L: Even faster running requires a dramatic shift; you'd been breathing as fast as possible, so to get even more air, you have to take deeper breaths, and this requires slower strides, which in order to keep pace, become longer. P: Lastly, there's a few seconds you can run without any air at all and you'll be breathing just because it's more comfortable than not breathing.
That's six kinds of running, which can be correlated to all of the terms others use. For each race, you have to decide how long you'll be running at each of those levels and then you can train specifically for each. If you're running 100 miles, only the first one's going to be very important. Dividing up shorter races becomes more difficult. As a rule, here's how long you can run at each level:
P: 5-15 seconds
L: 30 seconds to 3.5 minutes
M: 2-10 minutes
N: 20-60 minutes
A: up to 2-3 hours if not taking in food, 4-6 if eating.
U: several hours or even days
So, if you're planning to run a three hour marathon, you could do it all at A level if you're a world-class athlete; but if it's a personal best, it'll probably be 2 hours at A and nearly an hour at N, with perhaps a bit of M or L at the end, and training would then involve being able to run as long as possible at race pace staying at A level and trying to run for an hour at N.
A world class male 5K runner would only need to race at M,L and P: 10 minutes plus 3.5 minutes is more than the current record.
I was planning to train for a mile, which would require about even amounts of M and L and that's what I was going to focus on doing in training. I started improving rapidly by doing that, but I lacked the foundation of training that would allow me to continue running that way without getting injured.
So... for me, it's back to doing "base training" of slower miles to get prepared to do more faster running. I'm breaking the rule of training specifically.
Added: All of the descriptions of breathing patterns only work if, like me, your training is completely limited by oxygen uptake, which is unlikely for most people reading this (and really unlikely for ultramarathoners). If you're limited by anything else, you're more likely to recognize training levels by how they feel to your muscles. Not personally knowing what this is like, the best I can do is an analogy to weight lifting. "P" level is like one lift of maximum weight, where your muscles feel like they might explode. "L" is like the burn one feels after a few repetitions of heavy weights being lifted slowly. "M" is more like what one might feel if trying to lift moderate weights several times very quickly. "N" might be more of the "this is going to make me ache tomorrow" feeling one gets when lifting light weights for several sets. "A" and "U" don't correlate well to weights.
Big change in life today
2 days ago