Because:
Your muscles only have a finite supply of glycogen (I think, energy basically) and if you deplete it by doing big runs before your main "big run" then when it comes to the main run your muscles won't have as much energy and you will find it hard. That's why all training programs have a "tailing off" period before the main event.

If you aren't fit enough to manage the 10K by now, then it's basically too late, and try to cram in extra training before the event will just make you feel worse.
(Bazongaloid, Thu 29 Mar 2012, 13:43, 1 reply, 13 years ago)

I was fit enough for 10k it would seem
I'd just not run it before
( sporters I’m sincerely gratitude to you, Thu 29 Mar 2012, 13:59, Reply)