The Church of the Long Slow Distance (LSD) that all runners attend services at each weekend. 20%-30% of your weekly mileage should come from your long run (i.e. someone running 40 miles per week might do a 12-mile long run (30 percent of weekly mileage)).
Benefits from running long.
Key physiological adaptations that occur in the body during a long run are:
When you run long, you increase the enzymes within your muscle cells and grow capillaries/mitochondria. This allows for more oxygen to be delivered to the working muscles. You also strengthen your muscles, tendons, and ligaments in the process. Physiological adaptations like these will even help you in shorter races like the 5K; as it is still primarily an aerobic activity. The stronger your muscles, tendons, bones, and ligaments become, the more efficient you become in race-specific training.
Other benefits from your long run:
The body becomes proficient at burning fat. Fat is the body's optimal source for fuel. We have a lot of stored fat (compared to carbohydrates which we burn through quickly).
Muscle and liver glycogen (the major forms of stored carbohydrates in the body) are more effectively amassed and utilized.
The maximum amount of oxygen an individual can use (their VO2 max) during intense exercise will increase. The more oxygen we can utilize during hard sessions, the higher our VO2 max. With a higher VO2 max the more power we’ll produce during a workout.
You are building a really good aerobic base and teaching your body to work harder when it's tired. These practices will make racing come easier, build endurance, and increase your speed. Lack of endurance is the reason for slower times. Endurance translates to speed.
How far is far enough?
The LSD is performed once a week, usually on a Saturday or Sunday. Runners whom not looking to complete an upcoming marathon, should look at where their current level of physical fitness is at to know how far they should reach. Runners that are not used to running long, must slowly progress their running time up to 90 minutes. This will properly stimulate the body's adaptations. Once the body is adapted, they can continue to progress their runs up to 2 hours. Basically, progression is the key word.
While you are adapting to running longer, your attention should not be focused on pace. You want to run at a conversational pace, if it feels too hard then it probably is. Tempo runs and workouts are for building lactate threshold and long runs are not. They should be carried out at a comfortable and conversational pace, as stated. Once 90 minutes feels comfortable, try to finish the last 15 minutes of your long run quicker - pushing at the end.
How can you tell if you are ready to up your mileage and that you are adapting?
What you once though of as a long run, now feels shorter.
Your body/lungs recover quicker after running hills or after bursts of speed.
You begin to want to race longer events.