My planned workout this morning was a one-mile warm-up followed by eight miles at my goal marathon pace of six minutes per mile. Despite the 90-degree heat, all was going well until, with less than a mile to go, I felt a sudden, sharp twinge in my right hamstrings. Five years ago I would have doggedly finished the workout according to plan--after all, I was so close to completing it, and the pain wasn't completely debilitating. But years of living to regret such stubborness has taught me that discretion is the better part of valor, so I immediately slowed to a jog and took the shortest way home.
I am confident that, thanks to my caution, I will be ready for an easy run tomorrow and ready also for my next planned high-intensity workout on Friday. If I had froced myself to run that last 0.9 mile at six-minute pace, who knows? I might have completely ruined my next two or three weeks of training.
Bailing out of workouts due to unforeseen pain is frustrating. Believe me, I was cursing under my breath when I pulled up lame this morning. That's what makes it so hard to act sensibly and refuse to run through pain. But in the big picture it is much better to maintain a strict, zero-tolerance policy towards "red-flag" pain. You'll experience much less frustration and achieve far better results in the long run. I had to learn this lesson the hard way. Hopefully you won't have to (if you haven't already).