Here’s a trick from the world of ultramarathoning: Do long runs on consecutive days to get a big endurance boost.
As with running twice a day, this idea isn’t as crazy as it may sound. If you follow them with a few true recovery days, back-to-back long runs can lead to above-average endurance gains because of super compensation, wherein your body reacts to an extraordinary stimulus by preparing for its recurrence.
A decade ago I was preparing to run Maryland’s C&O Canal in a week. To get ready to average a marathon a day for a week, I did regular back-to-back long runs of 20 to 22 miles. After the first couple times, I had adjusted, and the second of the two long runs was no more work than the first. Knowing that I had to go long again the following day kept me from running the first day’s run too hard, and it gave me great motivation to do all the little things afterward to optimize recovery. I definitely felt more aerobically strong after a few weekends of consecutive long runs.
Back-to-back long runs work well in traditional training programs in the early to middle phases. For example, if you’re training for a marathon and have a few good long runs under your belt, you could do back-to-back long runs two of the weekends in the coming month. Keep the pace nice and gentle on both; just concentrate on getting the distance in. As you get closer to your marathon, you would want to move back to one high-quality long run per week for more specific preparation.