If there is one golden principal I stick to when training for ultramarathons, it is this: at least once in your season and no later than three to six weeks before your peak race, you need to do back-to-back long runs.
What is a back-to-back (B2B)?
A B2B is doing long runs on consecutive days. I generally run these on weekends. The second run is usually 66 to 75 percent of the first run. Mostly, my B2Bs are 20/15, 30/20, or somewhere in between. By the time I'm done, I've probably run 50 percent or more of my weekly mileage in two workouts.
Why do a B2B?
As an ultrarunner, your bread-and-butter training run is your long run. You maybe run one every week, or two every three weeks. And it is usually a hefty chunk of mileage - 25-33 percent of your weekly total.
But the B2B is a different beast. The idea is to stress your muscles on day two in a manner you're not accustomed to - working them hard by running a long distance while your legs are already tired. The benefits are two-fold. The first is physical. You're introducing a new stressor into your training routine, and from this stress you will gain endurance. The second is mental. By running a long distance when you are already partially fatigued, you simulate the latter stages of an ultra. Your brain takes over, and you train yourself to handle the stress that comes with those latter stages.
If you choose to do a B2B, each run will count as a quality workout. I generally do one or two quality runs per week, and rarely I throw in a third. A week where you are going to crank out 50 percent or more of your weekly mileage in two workouts doesn't need another quality workout. So skip the tempo run and do it another time.
How often should I do a B2B?
I usually do a B2B once every six to eight weeks. In Jack Daniels' training schedules, he recommends using four six-week long training phrases. The latter three of these contain can contain a B2B. If you're following his training emphases, a B2B is proper on weeks where long, slow distance is a focus.
This year, my peak event is the Sawtooth 100. In anticipation for it, I will do a 30/20 B2B six weeks out, a single 20-miler each week for the next two weeks, and then I go into a three week taper where my longest run each week goes from 15 to 10 miles of slow-easy running.