if you recover well, a build up is money in the bank, for seasons into the future.
after sickness, and you're not fully recovered, and you push an artificial schedule, so as to meet your mental notions where you should be and not listen to your body, then you are in trouble.
wait until you are feeling normal till you do anything hard.
what runners can do, is before they get way overtrained is to go easy for about 4 days. so if you have 2 shitty hard days in a row, knock the mileage way back, get extra sleep, eat right.
it's no big secret about proper recovery, but this is the mistake made by nearly 100%.
look up dorian yates and joe rogan, and the process of doing real damage and repairing properly, numbskull distance coaches can learn something, as well as could lydiard, the man who did not understand speed development at all, and confined the understanding to a ying yang, lydiard method versus anaerobic intervals. and his chickenshit and kick racing tactics. with that said, despite the gaping flaws, lydiard was probably the greatest coach of them all.