A good measure of how much work you’re doing as a runner is how much distance you’re covering. It costs just about the same amount of energy to run eight miles in 40 minutes as it does to run eight miles in 60 minutes; you’re doing the same amount of work--only the rate is different. However, the amount of work (mileage) that you’re performing represents only part of the stress to which you’re subjecting yourself. Slower runners spend more time accumulating the same mileage covered by faster runners, and more time on the road means more footfalls, more landing impact, and a greater chance for increased fluid loss and elevated body temperature. Thus, although mileage achieved is a logical starting point, it’s also useful to keep track of total time spent running.
Keep track of your weekly mileage so that you can use this record as a basis for how much of the various types of quality work you do and so that your training is consistent. Just as you use your current VDOT or (based on current racing ability) to guide your training intensities, you can use your current weekly mileage to set limits on quality sessions--but use time spent running to log points accumulated at various intensities of running.
In the case of weekly mileage, remember the principles of stress and reaction (principle 1, page 8) and diminishing return (principle 5, page 12) I discussed in chapter 1. Stay with a set amount of mileage for at least three weeks before increasing your mileage. This gives your body a chance to adjust to and benefit from a particular load before moving on to a more demanding one. When it’s time to increase your mileage, add to your weekly total as many miles (or one and a half times as many kilometers) as the number of training sessions you’re doing each week, up to a maximum of a 10-mile (15-kilometer) total adjustment. For example, after at least three weeks of 20 miles per week spread over five training sessions, your maximum increase should be 5 miles or 7.5 kilometers--1 mile (or 1.5 kilometers) for each of the five sessions you’re doing each week. In this case, you would be moving from 20 to 25 miles per week.
A runner who’s doing 10 or more workout sessions per week could increase his or her weekly total by 10 miles, after spending at least three weeks at the previous amount. Let a 10-mile (15-kilometer) weekly increase be the maximum mileage change, even if you’re running two or more daily sessions seven days a week. Another way of dealing with increases in weekly training load is to add to the weekly total the lesser of 60 minutes per week or 6 minutes multiplied by the number of training sessions you undertake each week.
I think that two hours a day of running is quite a lot, and it’s unusual for even elite runners to run more than three hours a day (about 30 miles a day for an elite distance runner). Remember that stress is a function of time spent doing something, and that’s why a 20-mile run is more stressful for a slow runner than for a faster one. It’s not just the 20 miles but the time spent completing those 20 miles. The increased number of steps can wear you down, and the extra hour in the heat or on slick roads can take its toll. To avoid overtraining and injury, slower runners might have to run less total mileage than faster runners.
This is an excerpt from Daniels’ Running Formula.