"Many runners introduce two runs per day before it is necessary. If your are preparing for races of 10,000 meters or longer, avoid double workouts until you have maximized the mileage that you can positively recover from in single workouts. Staying with longer single runs builds endurance and gives you more time for recovery between training sessions.Later, he affirms that
"When your mileage increases to the point at which your recovery runs last more than 50 minutes (or more than an hour during high mileage marathon training), then it is time to switch those days to easy double-workout days. Doing two runs of 35 minutes rather than one 70-minute run is easier on your body and enhances your recovery.(boldface mine) Have I taken his advice out of context? I kept the paragraphs intact enough for a reader to make his or her own judgement. I'd be interested if there are thoughts on this. Also I should mention that Pftizinger recommends against running less than 25 minutes per outing, leading to the curiously restrictive advice "always recover-run for more than 25 minutes, but never more than 50". But regarding the statements I boldfaced, I think Pfitzinger avoids direct contradiction by subdividing runners into the "less-than-10k group" and the "10k-or-more (usually marathon) specialists". In other words, as a marathoner you must focus on extending the single runs to increase your endurance; you must run your runs long, not more often. By contrast for those running shorter distances, frequent but shorter double runs are OK because they help with the intense/hard runs planned for later that day. Hence the notion of training 'long' versus training 'hard' should be treated differently depending on your distance.
But I question this underlying assumption. Elite runners training for a 5k vs a marathon show a perhaps surprisingly and generous overlap in total mileage. Brad Hudson, in his book Run Faster (the title is a nod to Maglischo's epic book), estimates most 5k elites run 90-110 miles per week, while elite marathoners train between 110 and 130 miles. If you consult with specific elites runners you will return with numbers well outside even those broad bands. In fact you will find mileage numbers well outside those bounds. Sebastian Coe, an 800/1500m specialist, trained at more than 120 mi/week (according to Canova), while Ron Hill an elite marathoner of the same era ran the 26.2 miles in 2:12 after training 70 miles per week (cf. Noakes' Lore or Running). Jerome Drayton, still Canada's marathon record holder, did up to 190 miles per week.
I claim, in contrast to Pfitzinger, that one cannot subdivide how one defines 'long' training runs based on how long the race is you vie for. True, experienced runners have a different definition of a long run is than do beginners, but that is a trivial statement; a true beginner will find just 15 minutes, not even 25, more than enough. Marathoners train using long runs as often as do many runners of shorter distances. All marathoners choose to do a long run either one per week or one every other week. As a 10k runner, I mostly did one long run per week, the usual that's recommended. Many elite middle distance runners simply avoid long runs, though barely. El Guerrouj trained by running an hour at a time, twice per day, every day. Had he stitched just one of those days into a single two hour run (and he would have, if not for back issues), his training would have been difficult to discern from a marathoner's to all but the most experienced coaches.
Point is, running is deceptively complex and there are many rules of thumb to be weary of. Choosing such "simple" guidelines for deciding on whether or not to run "double days" will create more problems than it can ever hope to solve.