"Usually follow hard days by easy days. Never have three hard days in a row".
My experience was that as I got very fit I might feel good after a hard day and feel like repeating the effort the next day - a faster than usual 15k run; a quicker interval session; whatever.
I usually got away with two hard days in succession. But a third was a recipe for disaster: even though I could produce another quality effort, then I would pay for it: along would come a severe cold or painful joints or unusual fatigue.
So if I ran well and felt great for two days in a row, I learned to treat that as a warning sign: "Slow down! Have a rest day!" rather than an indication that I could keep pushing myself.
I suppose you think it strange that "no symptoms" could be a warning sign. But - it must have paid off - I enjoyed fifteen years virtually injury free while maintaining a high level of performance and clocking anything from 100 to 180 km per week.
So - if you feel really good, that could be when there is the most danger of over-training. Remember to back right off at least every third day. Remember that rest is an essential part of training; rest times are the times when the body actually recovers and new levels of fitness are built in. You don't gain fitness during hard training; you gain fitness because of hard training, in the rest period that follows.
Training Principle #7 - Usually follow hard days by easy days. Never have three hard days in a row.
Long Intervals @Stromlo
6 hours ago