Yet you really have no realistic idea, either way. You must also think that Pete Pfitzinger overstates the importance of a “7th day of running” rather than cross-training.
The article posted by SwampTiger is a good one, and the brightest gem among what was quoted is as follows:
“The improvement, however, will not be as large as if you had increased your mileage.”
There seem to be some misconceptions floating around regarding base training, recovery runs, cross-training, rest, and mileage. First, recovery runs and base training are not mutually exclusive. In comprehensive base mileage training, some training sessions will be harder than others as it is important to vary the effort/pace between training sessions rather than keep it essentially the same for every run. Also, the noted problems with an easy recovery pace could very well be related to gait or form issues. FYI, the bit about glycogen possibly being depleted enough after one or two workouts to trigger fat-burning to any major degree during a recovery run seems rather tenuous, unless one goes for a couple of days without eating prior to and between workouts or said workouts are excessively long. Additionally, by opting to cross-train rather than take in recovery runs then one will be limiting overall mileage as well as the maximum length of long runs or the effectiveness of other training sessions. Jim Gerwick detailed this expertly in his article regarding the Hansons marathon training program in a recent issue of Running Times. As has been observed time and again, rest is an integral part of recovery; how much rest one will need for effective recovery depends on both the intensity and the duration of the training session, as well as other factors that feed into recovery such as hydration and nutrition. At some point, pretty much any runner will reap the benefits of a day of complete rest and one should certainly take a rest day of one feels like it — however, to get the greatest benefit out of training, the timing of a rest day should be determined by listening to one’s body (whether that is from self-perceived signals or by checking resting HR or whatever method one has found to be effective) rather than simplistically heeding a routine rest (or cross-training) day just because it is in a prefab training plan. Also, if one never tries any differently than the latter, then one will never know for certain if one’s fears have any basis in reality or not. Each individual definitely has a set mileage maximum. However, that maximum will not be reached except by a sensible build-up unhindered by mental limits; that maximum also can change with experience and accumulated training as well as with age and/or disease — in other words, recovery rate can both improve and diminish over time. It is doubtlessly true that cross-training can allow a person to expand one’s aerobic fitness beyond their maximum mileage limit, I know a top European athlete who did this in college and would incorporate 2-3 mornings of swimming each week once he was training at the maximum mileage and intensity load that he knew his body could carry. However, I have seen more than enough accounts of the training and racing that most runners who post to internet forums have done to have doubts that more than a few have ever made an honest and truly sensible stab at reaching that maximum mileage and intensity load. It seems that most who tout the virtues of cross-training as a part of their own training routines have not bothered to explore their maximum limits in earnest and therefore – as Pfitzinger indicates – end up short-changing their fitness. It is not wrong to err on the side of caution, per se, but it also is not congruent with optimal training.