Which comes first? The "superb conditioning" or the "semi-pro weekly mileage"?
I think they form a benevolent spiral where each feeds off the other to my full benefit. I feel pretty sure that, if I were to go out hard for every run, I would have crashed and burned long ago!
I think we're saying the same thing. As conditioning improves, one can go farther (daily or weekly), which further improves conditioning. That was my experience cycling. Last year my legs were the limiting factor that kept me from increasing my mileage. Now the limiting factors are time and motivation--I've found my legs seem able to adapt OK to increasing mileage now that I've learned how to maintain them pre- and post-run.
That's why I've halfheartedly tried to speed up the process by incorporating hills, intervals, tempo runs, like the pro sites suggest, because I don't have the time or motivation to run high weekly mileage, yet I would like to get faster/improve my conditioning. I don't know of anyone who recommends going all-out every time. The ratio seems to bounce between 80-20 and 60-40, easy mileage to harder running, at least on the few pro-style sites I looked at last year. And it's my sense that the higher the mileage, the more one runs easy. And vice versa; the less one runs, the more harder runs one can or should do. I read somewhere that Ryan Hall reduced his weekly mileage from 120 to 100 mpw so that he could fit in a few more 'quality' (his term, not mine) runs, like the Kenyans do.
So, applying this idea to a lazy recreational runner like me (while adjusting the mpw of course), I should run harder if I run fewer miles per week, and run easier if I run more miles per week, given my goal of improving my running fitness at a faster rate. That's the experiment I'm conducting right now--trying to find the right mix that works best for me. Latest idea: either (1) just run 3-5 easy miles each day, with maybe one day for interval work, and one long run per week, or (2), have 2-3 quality runs per week, one long, slow run, and on the other days rest or run 1-2 easy miles run-commuting.
Anyway, with your high weekly mileage, I wouldn't think you'd ever need to do intervals or something unless you wanted to squeeze out a little more performance potential. Since you don't race, why bother, right? Likewise, I don't really see the point of a HR monitor if you've already mastered aerobic running, but we don't want to get into that now, do we?