I read this article from active.com
Hansons’ philosophy (olympic coach), says feeling fresh in training is overrated. Success at 26.2 miles requires “teaching your body to run fast on tired legs.” Starting each workout with residual fatigue prepares runners for the physical and mental challenges of a marathon’s final miles. Treat each session as equally important so you won’t be tempted to take it too easy the day before a long run or hard workout.
Might as well read this too – Fight excuses that stall your training