I have heard both 1 week and 3 weeks before a workout has any effect on actual fitness.
Does anyone know more about this? I imagine there is a difference in "time-until-effect-time" between a vO2max-workout, a threshold-workout and a long easy-run as they target different physiological systems(?)
There really is no answer to this question. Start with it being doubtful any single workout will make a measurable change in fitness.
CNS changes take effect within a few days
Increased capillary growth, less than a week?
Increased blood cells, more than a week
Emotional/placebo effect, immediate
Increased mitochondria would be a few weeks days or a week
Of course this is all balanced against how long it takes you to recover from the workout.
This is actually very interesting I too have wondered this. I increased my mileage from 50mpw to 70mpw 2 months ago and only have started recently feeling the fitness boosts.
Within a day or two changes are well under way. You can easily verify this by repeating a workout two days later. You will be better at it already.
Technically a warm-up even increases your fitness.