Twenty years ago, the day after a long run was one of the days of the week I was least likely to double. I slept in, then ran 8 to 10 miles after work. Now that I’m in my midforties, the day after a long run is one of the days I’m most likely to double. Because I’m used to doubling, two runs of 30 to 40 minutes each result in less total fatigue for the day than if I were to run that much all at once.
Similarly, I’m not alone among aging longtime runners in finding that following a hard workout day with a day of a 4- and 8-miler takes less out of me than doing one 10-miler that day. Because of age, there’s a greater energy cost associated with that single longer run. For one thing, that 10-miler takes me 10 minutes longer than it used to, so it’s more likely that toward the end of the run I’ll be fairly fatigued and my form will start to deteriorate. As a result, the day is more likely to make me more tired rather than helping me recover from the previous day’s hard work.
You don’t have to wait to be my age to get this benefit from doubling. Once you’re accustomed to running twice a day, doing so on your recovery days is a great way to make sure they serve their purpose.