In reply to iusedtoclimb:
Miles is the most proven way.
But that doesn't work for everyone.
I always followed a high mileage marathon schedule, long runs every few days. Lots of quick running and Lactic threshold/tempo.
My best year was 2013 and I ran 90-100 miles a week all year and was probably in around 2:30 marathon shape. I could run 50-60 miles sub 7:00 min miles, and that was lots and lots of 12-18 milers.
Most of your physiological gains come after around 80-120 minutes of running. As many of them as you can. I think people get too obsessed with long runs rather than lots of quality longish runs.
However not everyone can tolerate high mileage. In back running around 80-90 miles a week, I'll probably do 100 miles this week and have forgotten how tiring it is. Lots of couch time and afternoon snoozes. I'll get up and do 10 miles before breakfast, have a mid day snooze and do 10 miles early evening.