Most coaches feel that once you reach 16 miles, you're in long-run territory. That's the point where the psychological and physiological changes kick in. Twenty miles is the peak distance used in most training programs in America.
Some coaches prefer to keep track of the long run by time rather than distance by hours rather than miles. Your time goal for the long run should approximate the total length of time you expect to run in the marathon itself, without worrying about distance or speed. For example, if your marathon time goal is three hours, you should probably do at least one long run of close to three hours.
The exception: If you're a first-timer with a goal of four hours or slower, you shouldn't do a long run of that length. It's too risky. Instead, do one long run of at least three hours, but no more than 3:30.
2. How fast should you run during long runs?
Speed is of limited importance during long runs. If you're looking for a specific pace for your long runs, aim for between 30 to 90 seconds slower per mile than the pace you expect to run in the marathon, but even slower is fine. Not every long run needs to be done at the same pace, nor does the pace within each run need to stay the same.
First-timers in most training programs run about the same pace they'll run in the race, because they choose conservative time goals to guarantee they'll finish.
Experienced marathoners who run long at race pace risk both injuries and overtraining. While you need to do some running at race pace to condition your muscles to that specific pace, this is best accomplished during midweek speed workouts.
3. How many long runs should you do?
If you're a first-time marathoner, you should probably only do one long run of up to 20 miles. Most runners who faithfully follow their marathon training programs jump from a 20-mile training run to the 26.2-mile distance fairly easily.
But finishing that first marathon and racing marathons are two different beasts. Experienced runners don't need to run 23-milers every weekend, but they probably do need to do between three and six long runs of 18 to 23 miles in their marathon preparation.