I've kept track of all of my training runs since last year. Nothing fancy ... just an Excel spreadsheet where I write down what I've done every day during my training for TNT marathons in 2010 and 2011. I'd really noticed, near the end of last July, that my body was taking a beating. While I'd felt generally pretty strong about my training last year, I'd realized, looking back,that my peak training month was July. That's not a good thing for an October marathon.
I'd ended up running 16 times for just over 100 miles last July. I remember feeling pretty burned out last summer, and the numbers confirm this. July 2010 saw me exercising almost 6 days a week (25/31 days combined), between running and cross-training. August, however, was much worse. I had more cross-training days (10) than running (9), and because I ran so many fewer days, I'd also ended up running fewer miles.
In other words: July 2010 was my peak training month for two October marathons. No other month had more training runs or miles. That's WAY too soon.
And in the months leading up to my marathons in 2010, there was a precipitous drop in both days run and total mileage.
This year, I've made a conscious effort to keep my runs, cross-training, and rest days in good balance. Generally, I'm aiming for 13 or 14 days of running each month (roughly 3-4 days per week), another couple of days each week of cross-training, and another couple of days of rest. Again, generally speaking, this should equate to, say, 14 days of runs, another 8 (or so) days of cross-training, and about 8 or 9 days of rest. It's all about balance. When in doubt, drop the cross-training for more runs.
So far, so good. No injuries. No burn-out. Just slow and steady as I've been adding the miles.