It generally happens when I've been out there for hours—when I've exceeded my training and everything hurts and the only reason I'm walking is to avoid stopping altogether.
I'm not sure why this is true, exercise physiology-wise, only that it is.
So, tempting as it is to slow and stop and sit and finally call it a day...I walk faster for a few steps then jog a few steps then run at best possible speed.
With very few exceptions, this tactic has always gotten me across the finish line.
***********Recent life (and death) events have me feeling a frequent undercurrent of dread. It's not inside the wire, but it's always near, testing my defenses. It prods here, there, then retreats for a while.
The next day, or the next, it's back. Testing. Pressing.
As is always the case during such times, it hurts less to run.
Run-therapy dependably shores up my defenses, chases away the dread, and buys some breathing room. It's temporary...but it works.
So I run.
Best possible speed.