The present study showed that speed endurance training reduces energy expenditure during submaximal exercise, which is not mediated by lowered mitochondrial UCP3 expression. Furthermore, speed endurance training can maintain muscle oxidative capacity, capillarization, and endurance performance in already trained individuals despite significant reduction in the amount of training. -journal of applied physiology
MY COMMENT: For a 4-wk intervention period, SET (speed endurance training) replaced the ordinary training (45 km/wk) with frequent high-intensity sessions each consisting of 8–12 30-s sprint runs separated by 3 min of rest (5.7 ± 0.1 km/wk) with additional 9.9 ± 0.3 km/wk at low running speed,
So when you take time off, instead of a total layoff, do an easy warm up and 10 x 30 seconds hard, followed by a cool-down. As long as you don't gain any extra weight, you should maintain a high level of fitness and have a much easier time of it once you begin logging miles again. In this study, runners averaging 45 km per week (28 miles per week) dropped to 5.7 km per week of hard short runs (3.5 miles) and about 10 km (6.2 miles) of slow running, or about 10 miles a week total.