As a runner, your body can only handle so much mileage. Some runners can pile over 100 miles a week. Some runners can’t run more than 3-4 days a week.
A better more emcompassing way to put it is that you can only handle so much time on your feet. Sure, some runners are faster than others and that’s why they can rack up 100+ miles at an easy pace, whereas if most of us ran the same amount of time we’d probably max out at around 60-65.
Back to the main point: While it’d be ideal to have you log 50+ miles while training for a marathon, many can’t quite hit that number within their reasonable best efforts, or their plan doesn’t ask that much. Even if your plan does, maybe you struggle for understandable reasons to do it: Hal Higdon might ask for 20 milers and 7-10 mile midweek runs, and maybe you don’t have the time to put them in… or your body simply gives out after 15 miles. Maybe the Hansons want you to run 6-8 miles six times a week, but there’s no way you can run six times a week.
However, as Jonathan Savage says, everyone running a marathon has to run the same 26.22 mile distance. Whether your longest run was 15 miles or 18 miles or 22 miles, everyone’s got to run 26.22 on race day. It doesn’t matter if you can’t handle the distance, the pounding that volume requires, in training. You’ll have to handle it eventually just like everyone else.
This doesn’t mean you need to do 20-26 milers in training to be ready. Some runners certainly can do that, sure, and they’ll usually be ready on race day. But while I do think it’s important to develop the aerobic endurance to go no less than 2.5-3.0 hours without stopping… what you do on the other days of the week can be far more flexible.
With one key caveat.
Your body not only has to be aerobically prepared to run long, but it physically must be prepared to take the pounding of that much continuous running. No matter how much you run or how you run, you must develop the physical strength to handle the 3-6 hour pounding. And that simply will not happen on a lighter running schedule by itself.
I also don’t think speed and tempo work is anywhere close to enough by itself. You’ll develop solid ability to run a 10K or something, but that won’t fully prepare you to handle hours of pounding and aerobic demand.
The successful marathoners I know and see all tend to have one other common denominator aside from just running a lot, running regularly and eating/sleeping/recovering well.
They strength train.