All that matters is duration. I still record miles, but I give more weight to how long I'm running rather than how far, and structure all my runs by duration. The way I see it is that if an elite is covering 20 miles in a little over two hours at an aerobic intensity, then why should hobby-jogger me be running more than 2 hours for a long run? If I run by mileage and go the full 20, it'll take me aprox 3:30 to cover the distance at an aerobic intensity. I'll probably make nearly twice as many footfalls. The body feels the impact of time on your feet, not the distance.
Which brings up a related point. An elite marathoner runs a marathon in a smidgeon over two hours, and my friend runs a marathon in 4 hours. There not really running the same race. One is running a 2-hour race at best effort, and cover 26.2 miles, and the other is running a 4-hour race at best effort and is covering 26.2 miles. Some people take 5-6 hours. Bigger impact on the slower runners.
Imagine a world in which all races that allow different levels of ability to participate are defined by duration instead of distance. Instead of a 26.2 mile marathon. The race is a 2-hour race. You go as far as you can in 2 hours. In terms of impact on the body, everyone is running the same race, approximately. Of course, this is just a fantasy as for many of the established marathons, this would be a logistical nightmare to stop the race at 2-hours. Fun to think about.