A friend of mine asked me if he "could run a workout after yesterday's long run?" My answer was "yes," but it induced the broader topic of scheduling workouts during a multi-week training block. 

At a macro level, it takes about two weeks after a workout for your body to gain incremental fitness. Therefore, your last hard workout in a training block should be two weeks before your goal race. During the interim two-week period, workouts should keep your legs sharp and poppy to help your body remember how to run at a quick pace. Use this fact as you design your annual training block. 

When scheduling a given training week, muscle soreness becomes the limiting variable. In general, a workout creates muscle soreness two days later. I wager this rings true for you from experience. There are two ways to schedule workouts with this knowledge: First, you can space workouts with two days of easy running between. Alternatively, but less preferred, you can space workouts with one rest day between. 

But life gets in the way of training plans all the time. What happens if work of family commitment prevents you from following the ideal workout spacing? Or if your training plan calls for three workouts and you start running out of days in the week?

Running back-to-back workouts can make sense in these special circumstances. Muscle soreness will not have peaked the day after the first workout. Fatigue will be present, but learning to run on tired legs has physical and mental benefits for race-day. You shouldn't schedule sequential workouts every week, but the occasional duo can help your overall running and bail you out of scheduling conflicts. 

Give it a shot and let us know what you think.