Hey Howie,
Good to see that you're digesting a lot of material here (and hopefully others are getting ideas from this thread, too.) Let me clear up some things.
The very race-specific workouts I described are taken from coaches who coach athletes as much as maybe 2 minutes per mile faster than I am. So they are designed for very fit runners at the top of their games, which is to say, immediately coming from a 'Special' phase (in Renato Canova's words) which may be similar to what you're doing now. Even given the talent and potential of some of Canova's (or his protoge's) athletes, they will take 2 easy days before and often even more after a workout such as I described.
So I don't pack in any more hard running than anyone else, but in fact, probably less during the 'Specific' (Canova's term) phase. I'm also probably twice the age of most of you reading this and have found that I need more recovery than many of you. What I'm saying, boiled down, is that a sequence such as those workouts described might take about 4-1/2 weeks ending 1-1/2 weeks before the target race. So, it describes the last 6 weeks.
There is, of course, other running, but those are the hardest workouts and many of the other days are just easy recovery running. Gavin Smith, an English-as-a-first-language Canova disciple has written about what he calls modulation, which is the dissimilarity between different days in a week or month. He says that modulation will increase as the race gets closer and training progresses toward more specificity.
This means that in the 'Fundamental' phase (Base-Building) there are noe extremely hard days but no extremely easy days either. You don't need the latter if you don't have the former. You can fit in more decent workouts when none of them are so hard that it ruins the week. A lot of days are medium-hard. Ultimately, the athlete arrives at a fitness level at which he can complete workouts such as the last couple I described. Each of these is hard enough that the week isn't full of several medium to medium-hard days. There is a very hard workout followed by easy recovery running until ready for another very hard effort. The modulation, or dissimilarity between (hard vs easy) days is huge.
Well, that's an inexact simplification. If you took a month or more to do what I described you'd probably slot in a couple medium effort days but a lot of them would be whatever your easy shakeout run is. The point is that you wouldn't be tempted to hammer a blazing track workout out 2 days after 9 miles or 15km at HM pace. Maybe you'd do 2 really easy days then a moderate fartlek or diagonals then another real easy then something hard again.
I got all this from Canova 101 in RT (it exists online), Gavin Smith's blog from just a couple years ago, after he was in Kenya working for Renato, occasional other articles (RT and online), but primarily from threads on this Board. I can give links. I wish to stress that none of the ideas are mine. The 4-3-2-1 mile is taken from JT Daniels, for example, although Canova runners will do 6-5-4-3-2-km according to Smith. The mile/2km repeats and 10-11 mile progressive tempo have been recommended by other posters. The ultimate race-specific tempo is not grounbreaking either: what serious marathon doesn't try twice that distance (15-17 miles or 26km) at marathon pace? Wouldn't a serious 10km runner do 4-5X2km and a 4 mile race pace tempo?
One of my favorite threads is Renato relating Mosop's preparation for Boston starting New Year's 2011. The final phase will seem unusual for the very reasons you mentioned in your post above: there are such hard workouts that recovery may become problematic and trying to put in much more than one good day a week could get you into trouble. I took shorthand notes of this specific phase onto a small piece of paper that stays with my own training log so I see it regularly. Here is a paraphrase:
A half marathon race (his only tune-up race) at exactly marathon pace ~1:01:30. I believe this is 6 weeks out .
A week of not much running then a moderate fartlek or 2 but no real hard workouts.
A 40km run that Canova says would be about a 2:10 marathon effort if done on a flat, paved, sea-level course with lighter shoes. Mosop set the course record on this road, which is saying something given the role call of runners Renato has taken there. This is 4+weeks out from Boston.
5 easy days. Well, one of the middle days was a moderate fartlek.
10X1600 at ~4:30 pace. Not real marathon spec but more half marathon. Several easy days.
8X3km hard, 1km moderate in between, for 31km without much rest. I don't remember the pace. Very marathon specific.
3 easy days if I remember. My notes aren't in front of me.
25km hard. I believe this was sub 4:50 pace, or maybe a little slower but with altitude around that effort. Marathon specific.
3 easy days.
The 'Alternator'. 12X1km hard, 1km moderate (but not much slower than the hard). 24km total pretty hard running. Pretty marathon specific, alternating between slightly faster than race pace and a little slower than RP.
One mellow fartlek a few days later, then taper. I may have small details off (this is from memory) but the point is, it really influenced me to try to do several very hard specific workouts immediately prior to a peak race - with, of course, a lot of recovery in between these. You can add a whole lot of time onto each, a minute per mile or whatever you need, and you can adjust for a half marathon or 30km or 20 mile race or whatever. In middle-age I may need even more recovery than you, but you see, you can adjust accordingly. The concepts are sound. Good luck, Howie, and keep us posted!