This article is from the running Beginners' FAQ, by Ozzie Gontang with numerous contributions by others.
Maybe the hardest part of running. You've had a hard day at the office,
it's lightly raining and you really don't feel like running. Believe me, 9
times out of 10, if you get out the door in your running kit you'll feel
fine after a couple of minutes, enjoy your run and feel better for it.
You have to learn to tell the difference between apathy and real tiredness.
One strategy is to tell yourself that you'll only do half the scheduled
run. If you really are tired, then you'll be able to tell in the first few
minutes, after which you should go home. If you stay apathetic, maybe
you'll do the half run, which is better than no run. Most likely you'll end
up doing your scheduled run.
On the other hand ... there is room for flexibility. If it's bucketing down
and blowing a gale, maybe it's better to leave the run until tomorrow,
unless you are one of those people who like running in wild conditions -
try it sometime.
 
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