Take a break in between periods of hard training
Ran just 3 miles tonight. Legs felt very sluggish, I think these may be times when your muscles are in a rebuilt phase. The first part of which is to tear them down before rebuilding them stronger than they were. These are the times when a lot injuries can occur. You could train really hard, and cause a start of the teardown phase of the rebuilding process which makes you stronger. The first part of this phase is teardown and if your training continues at the same rate or increases, injuries can occur. These injuries could caused by over working muscles which can not do what they ordinarily do can because they are in the rebuild phase.This is why you should takes breaks if you have been training hard. These breaks could be just easy, pleasant jogging for 3 or 5 miles. I have hard long days alternately with short easy days. Ultimately you must let your body be the guide.