Ever notice how fast time moves, with each year seeming to go faster than the previous year? Time spent running is also interesting—the second half of a run always seems to go faster than the first, and some runs seem to fly by while others seem to drag on. This changing perception of time when running may be partly explained by its relationship to effort, as Dr. George Sheehan once noted: "The faster we run, the longer it takes."
Runners tend to think a lot about mileage. Indeed, it's the number of miles you run each week that often defines your status as a runner. The more miles you run, the more you're validated. Other runners will ask you how much mileage you run and make judgments about you based on the answer you give.
The amount of time spent running, however, is more important than the number of miles—it's the duration of effort that represents the amount of training stress. A faster runner will cover the same amount of distance in less time than a slower runner or, to put it another way, will cover more miles in the same amount of time.
For example, a runner who averages a 7-minute mile pace for 40 miles per week is running the same amount of time as a runner who averages a 10-minute mile pace for 28 miles per week (280 minutes per week), and therefore is experiencing the same amount of stress. And that's what matters—the stress. If a slower runner tries to run as many miles per week as a faster runner, the extra time it will take increases the amount of stress and therefore puts the slower runner at a greater risk for injury.
The same is true for long runs—time is more important than miles. However, since races are a specific distance rather than a specific time, a faster runner doing a 22-mile run is getting more specific training toward a marathon than a slower runner going 17 miles in the same time. Since a marathon is 26.2 miles for everyone, regardless of ability, the race is more stressful for a 4-hour marathoner than it is for a 2:10 marathoner (assuming that both are running at the same percentage of maximum effort).
Therefore, a 4-hour marathoner needs to get used to running for a longer time than does a 2:10 marathoner to specifically prepare for the race. But this need to run for more time must be balanced by the amount of recovery time needed. In other words, if you focus solely on the number of miles, your long runs can get so long that the recovery time you'll need will increase dramatically and will negatively affect your next week of training.