I used to think the idea of easy mileage was a load of rubbish perpetuated by lazy individuals. I felt that the only true was to improve performance was to force the body to adapt to the demands placed on it through consistent hard training.
I was wrong!
Not only did I manage to give myself phases of achilles tendonitis, sesmoiditis and plantar fasciitis......my times also stagnated for nearly a full year.
Its important to read a few books about running, subscribe to a magazine such as.....Runners World or simply spend time learning from individuals on forums such as this.
The benefits of easy running include:
1) Improvements to the aerobic system. If you train at extended times for increasing mileages in lower rate heart rate zones, there is no two ways about it.....you endurance improves and your resting heart rate lowers.
2) An easy run helps you recover from a tough quality session. It helps keep the muscles supple, your weight nice and low and the blood in the right places for recovery.
3) Easy running helps you to work on form and cadence without the stresses of a tough workout.
4) A couple of days easy running helps also recharge you mentally (as well as physically) for the tough quality sessions.
5) As already indicated in my third paragraph, easy running can help with injury prevention.
If you are wanting to decimate your 5k and 10K times, why not try building up to a slow easy weekend run of 16 miles at 7:30-7:40 pace and see what it does to your times. I would also advise increasing the numbers of reps in your quality sessions. When I was at your level I was aiming for 12 x 75sec quarter mile reps with 90sec recoveries. I would also aim for 10 x 0.5 mile reps, but perhaps a little slower 2:50 pace?
Yep easy and steady miles are most important. I only do two quality sessions a week (Monday trackwork or hills & Thursday Tempo or time trial) and occasionally throw a few half mile reps into a longer run.
Too much quality work can hamper progress, bring on injuries and lead to you becoming stale.