An interesting thing to think about:
Is there an intensity threshold below which improvements will not occur? If you run 40 miles per week at 8:00 pace and you are quite fit (fit enough that 8:00 pace is about 55% of your max VO2), will running 60 miles per week improve your performance, automatically? Just because you increase your mileage, it doesn't mean you will become more fit if the intensity is below your personal fitness threshold (let us say 60-65% of max vO2).
Now, pay attention, because the following is tricky. Let us say that you increase your mileage to 90 miles per week, instead of just to 60 miles per week from your original 40 miles per week. Let us say also that you run the same easy pace that is about 55% of aerobic capacity. Based on the statements that no improvement occured when you increased your easy running mileage from 40 to 60 per week, we might therefore assume that increasing from 40 to 90 at an easy pace would create the same dismal result; no improvement. Ha! Now, I've got you!
Somewhere around 70-75 miles per week you started getting really tired and depleted in your muscle fibers that usually generate all the force for running at 8:00 pace. So, what now? You stimulate your body to use newer fibers to create force and they improve their fitness, which was never challenged before. As Peter Snell has said often, you stimulate fast oxidative fibers to contribute to the power output because the slower ones become exhausted as mileage rises high. So, in essence, you work on those fibers that are normally not called into play until you run at a pace about one minute per mile slower than 5k pace (we call this the Aerobic Threshold pace). In other words you end up stimulating your faster fibers to become better at using aerobic processes when your slow twitch, endurance fibers get tired and fall by the wayside at generating force to keep on chugging along at 8:00 pace.
Do you see now that when you run slowly, you have to run a lot of miles to fatigue fibers? This will result in improved aerobic performance and result in improved racing peformance.
Now, if we go back to 40 miles per week, but run it at 7:00 pace, which for this example is 70% of max VO2, well above the minimum stimulus threshold previously mentioned, we experience a different situation if we increse mileage from 40 to 60 per week. Instead of having no improvement in performance, we have a 30 second per mile increase in peformance, if we do this long enough to adjust to the new workload. This scenario is what happens to a lot of high school kids who run 18 minutes for a 5km race on 40 miles per week of training, then improve to 16:30 when they up their mileage to 60 per week. They had sufficient stimulus all along (they were 5-10% above the minimum threshold while running at an average of 70% of max VO2).
After such a wonderful improvement in performances, increasing mileage from from 60 to 80 miles per week, running at the same intensity, about 70% for an average, as you up the mileage seems like a great idea. Will there be a similar improvement of 30 seconds per mile? Probably not! Why? Because somewhere around 70 miles per week a runner's body stops responding in the same proportions as before when the workload was increase from 40 to 60 per week. Maybe improvements of 15 seconds per mile happen when one goes from 60 to 70 per week, perhaps just 10 seconds per mile, but when you move from 70 to 80 per week, you only gain 4 or 5 seconds per mile, perhaps none.
Once you reach 70 per week at the arbitrary intensity of 70% average of your max VO2, you have to figure out how you are going to gain improvements. Your choices? Increase mileage, but by this time, you realize that if you going to get more from your muscle fibers, you either gotta up the intensity or the mileage a lot. Realize what I am saying. Either increase the quality of some or all of your runs, some being smarter, theoretically, or increase your mileage by a big bump. Why a big bump? As described earlier, increasing 10 miles per week from 70 to 80 does not improve your performance more than about 4-5 seconds per mile and probably not at all form 80 to 90 per week. So, you gotta fatigue your muscle fibers a bunch in order for the mileage bump to work.
I say, therefore, you better bump it at least 20 per week, perhaps 30 per week, if you are not going to increase the intensity of training. This is critical to understanding why small bumps in mileage, assuming you are above the initial minimum threshold, will not improve performance, generally. Once you are past about 70-75 per week, you gotta bump it a lot if you are not going to increase intensity. Think about it and consider your own history of training and racing. Tinman