After buying new shoes because the cushioning on my old pair had worn out, I set out to do at least 15, hopefully 18, and just maybe 21 miles on this long run. I ended up doing 15.
Lap 1- I felt great.
Lap 2- started to feel a little foot pain, no biggie
Lap 3- still feeling the foot pain in my arches, a little discomfort in my legs
Lap 4- arch pain is getting worse, a little discomfort in the rest of the legs
Lap 5- arch pain unbearable, legs feeling like I just ran 12-15 miles on them.
I stopped after 5 laps, mostly because of the arch pain, but also because I was short on time. The arch pain was alarming - what if it was plantar fasciitis? The symptoms didn't exactly match that - so I figured it must be something else. The new shoes were the exact same model and size as the last pair, with the same custom footbed. Upon closer examination, I noticed a very subtle difference in the way I laced them. It was so subtle that I hardly believed it, but that was the only difference I could find. So I relaced them, and hoped that my next long run would be fine.
As it happens, I'm writing this a week after the event, and I just completed a half-marathon with no pain. It must have been the way I laced them. such a small, subtle difference. amazing.