At what point would you say one should go from singles to doubles? For me for example, it'd be easier to go from 8 miles a day on average, to dropping to 7/day with 3 miles per second run most days of the week, than to go up to 9/day. I like doubles because it makes me recover faster between runs, so another alternative maybe would be to double like above for a few weeks, then go back to heavier singles once my body recovers faster?