Great article here by Joe Friel looking at the importance of inteval type training for competitive endurance athletes.
"In a German study, 17 experienced runners steadily increased their volumes from their normal 50 miles per week to 105 miles per week over a four-week period (1). All of these runs were done at about marathon pace or slower (2mmol/L lactate). One year later they allowed the researchers to tinker with their training again. This time they nearly doubled the amount of time they trained at high intensity, over a four-week period again. With increased intensity they improved on four measures of performance from 5 percent to 17 percent. Increased volume produced no significant improvements in the same metrics."
Here is another viewpoint on the subject from peakperformanceonline written by Bruce Tulloh, European 5K champion in 1962. Bruce obviously is a huge fan of the low-mileage high intensity approach to training.
typical low mileage week of marathon training as described by Bruce Tulloh
Tues: warm-up, 8 x 800m on track (90 secs recovery jog) at 5k pace
Thurs: 10 mins warm-up, 2 x 20 mins at threshold pace;
Sat: 10 mins warm-up, 6 x 1 mile off road, (3 mins recovery) at 10k pace;
Sun: long run, 18 miles; 6 miles easy, 6 miles at marathon pace, 6 miles a bit faster.
Total mileage 41 approx.
Despite the evidence, if you like long, slow training, do not dispair! A study published in Medicine & Science in Sport and Exercise concluded the following:
"Our findings suggest that total training time spent at low intensities might be associated with improved performance during highly intense endurance events, especially if the event duration is ~35 min. Interventional studies (i.e., improving or reducing training time in zone 1) are needed to corroborate our findings and to elucidate the physiological mechanisms behind them."
Something tells me the true answer to the optimal training program lies somewhere in the middle.