There are a lot of components to this question, so let’s start with the concerns of decreased strength, power, and speed. These factors depend largely on your training regimen.
If you continue to incorporate speed work into your workouts, there’s no reason that increased mileage should harm your times at shorter distances. On the contrary, many runners have PRed at shorter distances on a slow, long-distance regimen alone, possibly because fast-twitch muscle fibers—the ones recruited for speed—are activated during runs lasting longer than an hour, a theory initially put forth in 1976 by exercise physiologist Phil Gollnick.
Integrating hills and plyometrics into your weekly mileage should maintain, or even increase, power and strength, as well as speed.
Bodily breakdown, from a muscular and cardiovascular standpoint, is where things get tricky. Unfortunately, many studies focus on the effects of marathon day rather than the months of training to get there. But those studies are clear: running 26.2 miles straight can have deleterious effects on muscles, elevating troponin and plasma CPK, indicators of muscle and cardiac damage, and C-reactive protein, an indicator of inflammation. Endurance runners face even greater risk of heart damage, outlined by the untimely death of ultrarunner Micah True earlier this year.
The best way to combat any negative effects on the heart, one study concluded, was to up training mileage to more than 45 miles per week. That 2006 study found that athletes who trained more than 45 miles per week for the Boston Marathon fared better post-race on several biomarkers for cardiac stress than runners who logged 35 miles per week or less.
The bottom line: The correct amount of mileage for you greatly depends on your biomechanics and genetics, but if your goal is to complete a marathon, gradually increasing mileage to 45 miles or more per week may help your body better handle the stress of running 26.2 miles straight through.
Coming next week, the answer to the last component of this question: How high mileage training affects muscle mass.