How long should your longest run of the week be? That depends on a lot of factors. Total mileage takes precedence over length of long runs when training for events shorter than (and in some cases, including) the marathon. Race pace is another critical factor to consider.

Keeping your training to date, race history and goals in mind, these guidelines will help you determine just how long you should be running for your weekly long run effort.

5K: Your long run should be a minimum of an hour at easy aerobic effort—once (if shooting for a time slower than 20 minutes) or twice (if aiming for something faster than 20 minutes) a week. For experienced runners, the time can be upped to 75 to 90 minutes, but anything beyond that might be wasted if gunning for a fast 5K is your sole focus. The long run should not exceed 40 percent of total weekly mileage. In the range of 20 percent is a good target goal.

10K: Your weekly long run(s) should be a minimum of 60 to 75 minutes at easy aerobic effort, once or twice a week. The same caveats as with the 5K long run apply concerning pace, but using 40-45 minutes as the cut-off for “fast.” Again, this run should not exceed 40 percent of total mileage for the week, but should be at least around 25 percent.

READ MORE: How to Shake Up Your Weekly Long Runs

13.1 miles: Aim for a minimum of 90 minutes at easy aerobic effort, twice every three weeks for your long run, although you can run this once a week. If you expect to take longer than 90 minutes to finish your half marathon, then you should definitely do this run weekly, but you should plan to vary the intensity. If you expect to race the half in 1:30 or quicker, your long run can range up to 2 hours once every two to three weeks. It should not exceed one-third of total weekly mileage, with 25 percent (in “up” mileage weeks) to 30 percent (in “down” mileage weeks) being a reasonable range.

26.2 miles: This one is a lot trickier because many runners can race a half without giving special attention to fueling and hydration (except on really hot days), yet virtually no one can race a full marathon, even under ideal conditions, without a fueling and race plan, and, more importantly, without having achieved certain physiological adaptations specific to very long events.

If you expect to finish the marathon substantially under 3 hours, your longest runs in any training cycle should be around 2 to 2-1/2 hours, although if you’re an experienced (at least 2-3 marathon finishes) high-mileage (around 70 miles a week sustained for at least two months) type, you can get away with longer, slower runs of 3 hours.

If you’re a 3- to 4-hour marathoner, you need to do runs somewhat commensurate with this duration of sustained effort, but you also have to account for how much more of a mechanical toll these will exact than “mere” 120- to 150-minute efforts. Try working up at least two to easy three to three-and-a-half-hour runs, spaced at least two and preferably three weeks apart, with the emphasis being time on your feet rather than pace and, vitally, keeping yourself hydrated and fueled throughout. Intersperse these with 8- to 10-milers at goal marathon pace.

If you think you’ll finish in between 4 and 5 hours, you’re unlikely to gain special benefits from running close to that far in training. Remember, your task is analogous to a 2:10 marathoner taking on a distance of around 50 miles. Focus on getting in at least three to four easy three to 3-1/2-hour runs, with at least two weeks’ rest in between. Don’t worry about your pace, because your everyday training pace is likely to approach or even replicate your marathon pace race if you’ll be out there for that long.

Pace, weather, tapering…all of these factor in a lot more than they do for shorter stuff—disproportionately so. The length and intensity also needs to be varied to get the most out of longer runs.

READ MORE: 6 Workouts for Running a Faster Marathon