Easy runs are the foundation that all other training is built upon.
For new athletes, the best way for them to improve is to simply increase the frequency and duration that they run at a nice low effort. Whether it's the beginning of your running life or the beginning of your marathon training cycle, it all should start with easy running. The speed and hard workouts come later.
What is an Easy Run?
Easy runs are not meant to build fitness as much to flush out the legs, loosen them up, and increase weekly volume. The pace should be just as it sounds, "easy". A nice guide is these should be in the area of 1.25-1.5 times your 5k pace. If your last 5k was done at 7 minutes per mile, your easy runs should be in the range of 10:30 - 12:15 per mile.
Purposes of the easy run:
1) Help build volume, which helps to increase or maintain fitness
2) Boosts metabolism to help burn fat, if weight loss is a goal
3) Helps keep the muscles loose, which may reduce DOMS from a previous workout
4) Increase blood flow, which may aid in recovery
5) Helps build aerobic fitness via increased mitochondria and cardiac output
This is the big question, how often should these easy runs be done? Lets say you run six days per week. If three days are key workouts, that gives you three other runs and one day off. Those three other runs should be a mixture of easy and moderate paces. Go by how you feel. If it's the day after your most difficult run of the week, perhaps you'll keep the pace quite easy. If you're feeling a bit energetic, you could do 3 miles easy and 3 miles moderate.
The important thing to consider is that easy runs help increase your fitness while at the same time allowing your body to recover from the previous hard workouts, that left you fatigued.
Generally longer and slower recovery runs are more beneficial than short and faster ones, because the longer and slower one adds more volume to your training. Use experimentation to find which types of easy runs work best for you. Remember, the goal should be to leave you feeling ready to run during the next hard workout!