For people who run almost daily, a long run should be between 20 percent and 30 percent of your total weekly mileage. So if you run 40 miles per week, have one run a week be at least 8 and up to 12 miles long.
Another general guideline is for your long run to be at least one-third longer than any other run that week. This guideline is more useful for people who run a moderate amount a few days a week. So if you typically run 5 miles four days a week, lengthen one of those to closer to 7 miles. It’s fine to cut back one of the other runs if you want to stay at the same weekly mileage. You’ll gain more fitness by varying the lengths of your runs.