Actually, the popular story around Tucson that my wife Michele and I habitually run 100 miles per week is somewhat of an exaggeration. I did get up to around 100 for a few weeks at the end of 2004, leading up to the Tucson half-marathon where I ran 74 minutes something. I was trying to peak for the Las Vegas marathon but injured my calf (not running-related, but it affected my running). For the first few months of 2005, I wasn't able to train properly, because of the injury, hence the "slow" result in the Tucson 5000. I have since run 16.52, but as that is slower than my average pace for my best ever marathon, I don't regard it as anything special.
In recent months, I have been averaging about 50 miles a week, but I am now back into full training and this week will be my first at 100 this year (including 2 speed workouts). I am planning to go up to 120 this summer, while training for the Twin Cities marathon. This is something of an experiment to see if a near-55 year old can sustain that type of workload and then race well. I guess the proof will be in the results of the TCM.
Michele (who also turns 55 this year) typically runs 85-90 mile weeks when in full training, although she also might go up to 100 this summer. She is a former Olympic marathon trials qualifier and is used to running high mileage.
Age is just a number. There are no rules and there are no limits, except for injury and enthusiasm.