When doing long runs, athletes often use ‘cumulative fatigue’ to train their fast twitch muscle fibres, which are recruited when the slow twitch fibres are fatigued. Most likely, all training plans have long runs at least one day following a fatiguing workout.
Wouldn't it be more efficient to do the long run on the same day after interval training?
For example, if we were to do 4 × 4-minute intervals, the total workout time would be around 45 minutes, including warm-up and cool-downs. So we should expect the long run to be around 45 minutes, instead of 90 minutes if we were to perform it on a separate day.