Another book that's got an interesting approach on this subject is Mastering the Marathon: Time-Efficient Tips for the Over 40 Runner, by Don Fink.
I've been loosely following his approach because I'm an over-40 novice-ish runner, and I'm very prone to over-training injuries, in particular because I have hyper-mobile joints. (For example, I can rotate my feet almost 180 degrees backward.)
Fink focuses more on quality than quantity and, like a number of other coaches, stresses 3 key workouts per week: Tempo run (a medium-length run at race pace), Speed run, and Long, Slow Run. What's interesting, though, is that he explains that you can do some or all of the Speed run and LSR through a combination of running and cross-training, in order to reduce the risk of injury. The only "run" that he says needs to be a "pure" run is the tempo run.
I've been following this approach for the year, and (knock on wood) it seems to be working for me. I generally follow a 9-day cycle (2 days on, 1 day rest, multipled by 3), so I do a long slow run only every 9th day (instead of every 7 days). My LSR is a combination of running and elliptical (usually I start in the elliptical to warm up for 10-20 minutes, then run for 30-60 minutes, then switch back to the elliptical and then try to end with 10 minutes of running). This allows me to build endurance without all the pounding. Also, by finishing with running, I do build up some experience running while "fatigued". I don't do a lot of speed work, but I usually do some once per week and either do it as a run or on the elliptical. I also usually do the elliptical or bike on days 2, 5 and 8 of the cycle.
In addition to the cardio work, I do a lot more core strength training than I used to, and I do that about 2 out of 3 days.
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