As a freshman in college I was fortunate to run for Furman University. After running 20-30 miles per week for my 2 years of high school track and cross country, I was in for a shock as we started the cross country season at 60 miles per week and peaked at 100! I stayed injury free and got better, a lot better. The problem, though, was that my progress peaked before the end of the season and I began to regress. My times were getting worse and I was feeling very fatigued and couldn't sleep. All classic symptoms of overtraining.
The problem wasn't so much the mileage, rather how hard I was running those miles. I was a freshman trying to prove I belonged on every run. Instead of taking easy days after hard efforts, I'd run recovery efforts as workouts as well. The result was a total breakdown. Since that season I have learned much more about training and the importance of the hard/ easy principle. That idea has been studied a lot lately and has been refined to reflect an 80/20 principle.
The idea with the 80/20 training is that 80 percent of your runs should be at that conversational pace. Twenty percent, then, should be at paces that are harder and will depend on what you are training for. (More on specifics in a training plan in another post). It may seem counterintuitive to run slow that majority of the time when you are trying to race faster. But, remember that all races from 5k above are at least 90% aerobic. The majority of those benefits actually come at these easier paces.
Back in Florida one of our most successful runners and I were chatting about a race she had just run. A PR of sub 20:00 for 5k. She said she didn't understand how a PR was possible since she had not done any speed work, just a lot of easy mileage. The reason was exactly what was discussed above, the 5k is mostly aerobic and she had trained that system to a T.
An article follows supporting these thoughts. Have a great week and happy running all!