A quick comment about higher mileage and a supporting lifestyle, relating to duration of higher mileage training.
During, and since running my highest mileages, I have itinerantly worked the spectrum of jobs, from farm labour to manufacturing, residential and heavy construction, to lab technician/statistician and technical writing gigs. This may seem really obvious, so it must be worth saying aloud: What you do outside of running, be it work, or spending time ferrying yourself to and from work, or caring for a spouse or parent, or drinking yourself into a stupor several nights a week, it all comes to roost in your ability to cope with and recover from the stresses of beating your body up on the roads.
It is no coincidence that many of these comments harken back to high school or college days, when time was (for most) aplenty, and responsibilities a little less so.
There are AAA personality type, work capacity monsters out there who work full time while going to medical school and knock out 100mi/week in singles year-round, without getting hurt, sick, or stale. They probably put on spring race series as well, have a couple kids, and a dog name Sprocket. These "people" should not be the guiding light of the general milieu; the line of thinking that you are the unique exception to cumulative stress, and a physical virtuoso (on top of being exceptionally good looking) has, at some point, gotten most ambitious distance athletes far into the gutter, limping, or out of the sport entirely. Most of us are not the uncrackable egg thrown at the wall of high mileage.
To put in the time (whether it is 10, 15, or 25 hours a week) and then stick around long enough to reap the rewards of that work in the form of results or fitness, one has to have a lifestyle that is somewhat accommodating to keeping the tank above empty. And not some of the time. All of the time. The ability to sleep enough, eat enough, keep the stress at home and at work down, they all factor in to your mortal self not disintegrating. And lastly, persistence and consistency are king and queen of volume. You see several "build up to higher mileage, then miss a year" stories from those who disobey these rulers. Obey, and go long.