Monitor the Signs
A concrete way to detect this unusual fatigue is to repeat a standard workout, like the 6-mile tempo that British Olympic 1500-meter runner Andy Baddeley does every Thursday. He always runs at the same heart rate (just below his lactate threshold), then he times how long it takes for his pulse to drop below 125 beats per minute after he stops.
Sudden increases in the tempo or recovery time signal that the body is struggling to recover, which means you should start the two-week maintenance phase of overreach training. To try this approach, start doing a weekly 3- to 5-mile tempo run at least four weeks before you ramp up mileage, keeping your heart rate between 80 and 85 percent of max after the first mile, and repeat every week until the overreach phase ends. You can also take your resting heart rate first thing every morning: If the seven-day average drops by more than three beats, that suggests you're overreaching.
The biggest mistake you can make after an overreaching block is to settle back into your regular training, letting the accumulated fatigue linger. Instead, immediately drop your mileage down to about 60 percent of your pre-overreach levels for one week, and dial back the intensity of your hard workouts. Make an extra effort to get plenty of sleep, and book a massage to address any minor aches. If you don't feel refreshed and raring to get back to full training after this recovery week, that's a serious warning sign that you may have overdone it. In that case, bring the mileage down to 40 percent of normal for another week and stick to easy runs (or try taking one of these 5 Walking Workouts That Help Your Running.)