But as I accepted my failure in the intended workout an idea crept into my mind. Instead of turning around at the halfway point I decided I would run a couple of miles farther at an easy pace to give myself time to recover, then dial it back up to tempo pace for the remainder of my planned tempo distance.
The result of this scheme was a negative split for the second half of the tempo "interval," and just like that, a workout that I thought was trashed ended up in the win column. I am not going to claim that this workout had all of the benefits of a full 5.5 mile tempo run, but it was a far cry better than limping back at a pace far too slow for tempo but too fast for recovery.
In the end I think this demonstrated to me the importance of flexibility in training. Not every day will be a good day and it's important to take what you can on those days. Listening to your body and adapting to its needs will always be better than trying to grind through a workout and ending up ineffective, or worse injured.