Conventional wisdom, or at least Hal Higdon, always preaches that your mileage should increase by no more than 10% a week to avoid injury. If I'm understanding it correctly, a short reader comment in the Letters to the Editor section of the newest Runner's World even suggests that this should be modified down to 5% for older runners. Sounds reasonable enough, right?
I've always had trouble with this argument. It would seem to make sense for high-level athletes who are running mileage at or near 100mpw, for sure. But the smaller the mileage, the more restrictive this rule is. Even if you're only (only!) running 50mpw, that allows you an increase of only 5 miles over the course of a week - not even an extra mile per run, let alone adding substantially to your long run.
Those of us who have fallen out of fitness may remember fondly our days of regularly running 35mpw, and working up to that from 10-15mpw could take 14 weeks. Three months. A long time - cautious, to be sure, but that's a lot of build up and a lot of waiting and not running for someone who just wants to get out there. And what about entering a training cycle? As you amp up your long runs in preparation for a goal race, it's necessary to also increase your midweek runs to have the fitness necessary for consistency in your long runs. This adds to your mileage, possibly holding you back at a time when the point of what you're doing is to push yourself. AND, this assumes absolute consistency. For me, the difference between my training schedule and my training reality is often measured in missed runs... which lower my weekly mileage.
I've always personally felt more comfortable listening to my body and going with what worked for me. Being cautious is all well and good, but sometimes throwing caution to the wind is advantageous. If you put in a high mileage week and feel good, there you have it. If you put in a high mileage week and feel worn down, you may be trying too hard. After a certain point, most of us work out a pretty fair idea of what our bodies can handle and what they can't, what our risk of injury is, and what it feels like to be at or close to the edge.
A sidebar in this month's Running Times backs me up (next to the guide to dynamic stretching on p. 28 - sadly not online). They argue, citing Jack Daniels, that the best way to increase your mileage may actually be to do exactly what I propose: just do it. Then, give yourself a few weeks at the increased mileage to see how it feels before readjusting. Working slowly up is not only unnatural (your body doesn't understand the base 10 system), but it doesn't allow your body time to adjust to the higher mileage before continuing to increase.
I do realize the irony of making this argument, given that I've had two stress fractures. The first was simple overuse: I went from never having run further than 12 minutes (gym class in high school) to running 30-40mpw in one eight week period during college. The second came, similarly, from doing too much too fast: I went from running consistently to training consistently, with speedwork and longer runs, without any care to the additional stress put on my body. Both times, I knew something was wrong and I decided to push through it.
Incidentally, there's also another article in the same magazine that suggests that the benefits of running are cumulative over time, that running-related gains in fitness over time make it easier to do more on less training. I like that.
On I went, out of the wood, passing the man leading without knowing I was going to do so. Flip-flap, flip-flap, jog-trot, jog-trot, curnchslap-crunchslap, across the middle of a broad field again, rhythmically running in my greyhound effortless fashion, knowing I had won the race though it wasn't half over, won it if I wanted it, could go on for ten or fifteen or twenty miles if I had to and drop dead at the finish of it, which would be the same, in the end, as living an honest life like the governor wanted me to. -Alan Sillitoe, "Loneliness of the Long Distance Runner"