In Reply To:
2 x 10 miles is not the same as 2 x minutes. All things being equal, the closer you stress yourself to your breaking point (without going past it), the more you will get out of your training.
How could you be able to do 2 x 10 mile and not do 1 x 17 mile? If anything 2 x 10 seems worse.
The very reason that we do long runs is the danger of them. We are trying to teach ourselves to run beyond that fatigue barrier, and, in the very long runs, beyond that glycogen barrier. But it is in those fatigued miles that form suffers. That overuse injuries are most stressed. Miles 10-15 or 10-17 are far harder on the joints, tendons, etc than miles 1-10.
And so in those time when I have been fighting off such injuries, I found that simply doing more miles (or frankly even the same miles) but breaking them up into two runs allowed me to avoid those miles that would knock me out of comission for a couple of days. The very next day I could get up and do another 10. Now admittedly I had no long run, but for marathon performance, I have found from hard won experience that weekly mileage trumps long runs every time.