Some seven months ago, my achilles' tendon gave me the impossible-to-ignore message that marathon training could soon be its undoing. Since then, I've encouraged myself to look more critically at several different aspects of my training. I'm looking not so much because I want to complete another marathon, but because I want to continue running. Walking with a minimal limp (from bones broken almost three decades ago) is a plus.
It's meant, at least for me, some changes to my workout schedule...some ways in form, but mostly in duration.
I firmly believe in the ten-percent rule; the rule (of thumb) which states no increase in distance, intensity (which I look at as average heart rate over time) or duration for an exercise activity should be more than ten percent over the measure of the previous three-to-four week period. There are several coach/author-types who have published this same belief.
But not all coaches agree. My friend Patrick McCrann is one of the few who openly disagrees. He writes that increases of any sort need not necessarily be at the "rigid" ten percent point. Some times it can be more; other times less. What is important, in his mind, is to adjust the stress once the body adapts to it, then keep it there until the adaptation is complete.
I understand his thoughts on the subject. Since we are all an experiment of one, we adapt to stressors, and the change of those stressors, in varied ways. Two eight-minute milers can run on a flat course for 30 minutes at a time and perhaps both feel the same physiological benefit/stress that we can measure (heart rate, respiration, fatigue, just to name a few) during that ~3.75-mile jaunt. Add three minutes to that run and one of the two may walk around the next day with aching joints; the other might be "itchy" for more distance.
Naturally, we can't isolate all of the factors which could cause differences in adaptation. Not without putting an entire population in isolation. If we could I am most certain there are entire populations we would prefer to isolate...most likely not our own.
When making those changes in stress, it's important to keep static as many of the other variables as possible for at least three weeks. If your body responds in a less-than-desired fashion, like an injury, it's probably a good idea to back off the stressor until the injury subsides. Injuries are usually the body's way of saying "I am not able to adapt to this much stress." Stay at that level for three weeks, then increase or change the stress as tolerated.
A little imagination, and close attention to your body's response to stress, is key to increasing mileage, duration, or intensity in your training plan.