Running 10 miles in a single run on a given day is certainly more strenuous than breaking this into two runs of 5 miles (or of 6 and 4, or whatever). Doubling may have side benefits such as increasing powers of recovery, but let's ignore that here. Then my question is simple: what would you say 80 miles achieved by doubling most days is roughly equivalent to in singles?
Equivalent mileage in singles vs doubles
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91.8 Exactly. Seriously. no one can give a precise number Eighty miles is eighty miles and you do want some runs with good duration in the mix. But you'll get better results with some doubles included.

70 miles on singles is 10 miles per day on average.
Now let's say you ran 80 miles by running 10 miles per day and then two days you ran a 5 mile double. Obviously that's more impressive because it's the same exact thing as the 70 mile week but also has two extra runs.
Most people don't double every day, it's typically 24 times per week they would double. I would say 80 miles with 3 double days is like 75 on singles. If doubling is so easy, why doesn't that 75 miles on singles person just run a couple more doubles and get that up to 85 miles?
Mileage matters and it matters a lot. If a 75 mile person would start doubling and run 95 miles they're going to get a lot faster. 
Running doubles is the best way to go if you're starting from scratch. The problem comes when you go from running 70 mpw in singles to 70mpw in doubles. Those two are not the same.

parkerjohn wrote:
The problem comes when you go from running 70 mpw in singles to 70mpw in doubles. Those two are not the same.
Exactly. Though I take HRE's point that of course one cannot put a precise number on it, I am interested in anecdotal evidence as to what people consider to be vaguely equivalent numbers. Thank you to the poster above who made an attempt to put some numbers on it! 
mileage conversions wrote:[/b[b]]
Running 10 miles in a single run on a given day is certainly more strenuous than breaking this into two runs of 5 miles (or of 6 and 4, or whatever). Doubling may have side benefits such as increasing powers of recovery, but let's ignore that here. Then my question is simple: what would you say 80 miles achieved by doubling most days is roughly equivalent to in singles?
Two five mile runs at sub6 per mile are certainly more strenuous than one 10 mile run at 9 minutes per mile or slower. 400m to 1500m athletes and maybe 3000m athletes must have double runs. Double running is how 400m to 1500m athletes fit in necessary workouts, plyometrics and weight training. 
not always true wrote:
mileage conversions wrote:[/b[b]]
Running 10 miles in a single run on a given day is certainly more strenuous than breaking this into two runs of 5 miles (or of 6 and 4, or whatever). Doubling may have side benefits such as increasing powers of recovery, but let's ignore that here. Then my question is simple: what would you say 80 miles achieved by doubling most days is roughly equivalent to in singles?
Two five mile runs at sub6 per mile are certainly more strenuous than one 10 mile run at 9 minutes per mile or slower. 400m to 1500m athletes and maybe 3000m athletes must have double runs. Double running is how 400m to 1500m athletes fit in necessary workouts, plyometrics and weight training.
Eh, 30 minutes at (lets assume) moderate is not more strenuous than 90 minutes reallly easy imo