"increases in volume should be done very gradually. many books permit a 10 percent increase per week. if you were to increase at that rate for an entire year, you'd double your workload every 7 weeks - expanding it by a factor of nearly 200 by year's end. obviously that's insane; programs that advocate a 10 percent per week increase are attempting to jump you quickly from, say, 20 miles to 25 miles, then hold you there for a few months, hoping you won't break down from the sudden change.
i prefer a slower buildup of no more than 10 percent per month. that way, you would go from 20 to 25 miles in a little longer than 2 months: running 22 miles a week in the first month and 24 miles a week in the second. the third month you'd take the final easy step to 25 miles. at that point, however, even this slow program requires a plateau: it's unwise to increase you weekly mileage by more than about 25 percent in any given 6-month period. even at that rate, you could go from 20 to 50 miles per week in two years - an enormous change.
if you're tempted to accelerate the process, the risk you run is a succession of nagging injuries. there are several parts of the body that have to adapt to the stress of increased mileage, and they do so at different rates. aerobic conditioning comes first, with muscle strength following closely behind. that's why so many people try to proceed too quickly: once your muscles and cardiovascular system have adapted, workouts feel deceptively easy. what you need to remember is that tendons, ligaments, and bones also need to adapt - something they do fairly slowly. bones are the slowest, and the potential injury, a stress fracture is the one i most fear in beginning racers.
stress fractures don't come from a single wrong step; the culprit is repeated jarring. i could give you one with a doctor's reflex hammer by tapping a bone over and over, never hard enough to hurt. tap-tap-tap for tens of thousands of repetitions until suddenly, the bone can take no more. in running, stress fractures come on similarly, virtually overnight. one day you feel as though you could run forever - the next, wham you have a hairline crack in your foot, shin, or hip. the best analogy is to flexing a coat hanger back and forth. do it enough times and the metal will fatigue and eventually snap. the chief difference is that given a chance, bones will protect themselves by getting stronger before anything goes wrong.
for the same reasons, you should never increase your base mileage while simultaneously adding speed workouts, hill running, or anything else that increases the difficulty of your training. increase the base mileage, then add the other workouts once you've attained your mileage goal."
("alberto salazar's guide to road racing" p. 40)