Regardless of the length of the race you are training for, all runners benefit from the long run. But how long is long? Here's an easy way to figure it out:
Your long run should be about twice as long (in time) as your normal length run. You can also determine your appropriate long run distance by making your weekly long run 20 -30% of your average total weekly mileage. As an example, if you run about 35 miles per week, your long run would be 10.5 miles.
According to Ed Eyestone writing for Runner's World, to avoid injury while gaining the most fitness, we should choose the calculation that yields the shorter distance - miles or time. Here are some examples:
5K // 12 - 15 miles or 1.25 - 1.75 hours
10K // 15 - 17 miles or 1.5 - 2 hours
Half Marathon // 17 - 20 miles or 1.75 - 2.5 hours
Marathon // 20 - 25 miles or 2 - 3 hours