Here, I'm going to try to break the marathon into parts and explain why slower runners have to train differently than fast ones.
In my previous series, I mentioned the theory that a race could be broken down into sections based upon energetics (the Scott post: here). One can sprint using creatine phosphate (P level) for at most 15 seconds, build up lactic acid (L level) for at most three minutes, run at maximal oxygen uptake (M level) for 10 minutes, run at anaerobic threshold (N level) for 60 minutes and aerobically at training pace for 120 minutes (A level). As one progresses at a constant pace, one's heart rate creeps upward until it hits maximum, so one moves from A to N to M to L in one very long run (P being reserved for the final kick at the end. If you see the clock read 3:59:40 at the end of the marathon, you'll probably discover that you can speed up!)
These different levels are very different percentages of a marathon race for runners of differing abilities. For a 2:00 marathon, 50% of the race is done at N, and only 40% at A. For 3:00, 60% is at A, 35% at N. As A+N+M+L+P is 3:15, another category has to be created for running longer than this, which is running slower than training pace and which I'll call "U." A 4:00 marathon is 50% A, 25% N and 20% U. A 5:00 marathon is 40% A, 35% U and 20% N. A 6:00 marathon is 45% U, 35% A and 15% N. Thus, as one moves from the fastest marathon times to the slowest, the priorities shift from N to A to U.
Here's how those numbers correspond to the miles of a marathon:
For 2:00, A is miles 0-10.2, N 10.2-23.3, M 23.3-25.5, L 25.5-26.2
For 3:00, A is miles 0-15.5, N 15.5-24.3, M 24.3-25.7, L 25.7-26.2
For 4:00, U is miles 0-5.1, A 5.1-18.2, N 18.2-24.8, M 24.8-25.9, L 25.9-26.2
For 5:00, U is miles 0-9.3, A 9.3-19.8, N 19.8-25.1, M 25.1-25.9, L 25.9-26.2
For 6:00, U is miles 0-12.1, A 12.1-20.9, N 20.9-25.2, M 25.2-26.0, L 26.0-26.2
The 4-6 hour marathoner will most likely disregard the L and M levels, as improvement in the other categories will render them inconsequential. The closer one comes to the limit of one's ability, the more important the latter categories become. If these runners do not start the marathon slower than their standard training pace, they will be forced to tack the U level running on the end of the race, which is a variation of "hitting the wall." Jeff Galloway's idea of incorporating walking breaks just intersperses the U level running throughout the run (and, as one can run faster than one can walk at this level, leads to slower - though perhaps more comfortable - finishes).
One can only run 2:00 at A at the slowest end of the range of paces that constitute A, so if one starts at too fast a pace, one runs through the A-N-M-L chain before reaching the finish, forcing one into the U category; this is another variation of hitting the wall.
The long slow training run becomes increasingly important as finish time increases. From 4 to 6 hours, the goal is to get to 18-21 miles very comfortably. A good general rule is that, if you notice your respiration, you are not running at A or U.
For the 3 hour marathoner, nearly half the race is done at other than A level, so training to extend the range of N, M and L is important. One needs to do speedwork specific to these ranges. A time trial of 20K will require all of these, for example. It is careful control of pace and effort throughout the majority of the race that is important - hills can quickly pull one into the next zone, so one needs to prepare for them - and then being able to do the final hard 2 miles without slowing. Because one is nearly at the full 2 hours at A level in the 3:00 marathon, it is important to run 2 hours often in training to find the pace one can comfortably maintain for that long (6:52/mile, being 3:00 pace, of course the goal).
For the 2:00 marathon, the pace of one's long runs will be more important than their length, as the comfortable A zone ends at 10 miles. The final 5K will be at M,L and P, so 5K racing ability comes into play; the first to break 2:00 will be able to run 5K in 13:00-13:15 (faster than this and he'll probably be better suited to the 5K or 10K distance).
I was once capable of 2:30, but had trained incorrectly and had no concept of these effort levels. I hit the half-way mark in 1:14:29, but fell apart shortly thereafter; I had already burned through N,M and L in the first 25K, having unknowingly trained well for 25K rather than the marathon. My long runs had all been at U level and when I thought I was starting at the comfortable A level, it was because I had done a ton of training at N and that felt "normal." I ended up slowing continuously to 1.5 min./mile slower than I started, finishing in 2:44 (still my 4th best time). Had I started just a little slower, I would've run about 2:35; had I trained differently, I might've been able to break 2:30. Instead, I abandoned marathoning for 18 years, as "it just isn't my race."
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