Not that hard to go too fast....... wrote:
The difference is only 8 seconds per mile between best pace for 20 miles and best pace for 26.2 miles. That's why it is so easy to "hit the wall" around 20 miles.
exactly, well said
it sometimes takes inexperienced runners/racers a few times to completely understand this, unless they have good coaching and "follow orders" in which case they might avoid experiencing the problem
people also confuse what I call the "LT Wall" with glycogen-bonk, which are two entirely different problems but both can manifest anywhere from 18-22 miles so they don't understand what is going on
if you walk or slow dramatically and then recover back to pace, it was a LT Wall and you were going too fast, the lactic acid overwhelmed your muscles and didn't clear out fast enough
if you have to walk and or have to slow to a crawl for the rest of the race and never recover, it was likely glycogen bonk and you ran out of fuel
under 20-22 miles you might never face glycogen bonk, depends how you eat
both problems solved by slowing down but of course it's a race and you want to go as fast as possible, hence using half-marathons to find your true lactic threshold and then do an educated guess as to your proper full-marathon pace (and fueling properly for a full, before/during)