“only 70 or so miles of biking last 18 days??”
as the race approaches, i think you have to ask yourself, “how much fitness is going to be gained from here to the race?” if the answer is, “none, or not much,” then i think your goal switches from gaining fitness to gaining freshness without losing fitness. no, there’s not much cycling in the last 2 1/2 weeks, but there was a LOT of cycling up to that point.
“not sure I understand the philosophy behind the planning from the race backwards”
everything in my life where there’s a target performance date is approached this way (not just training). for example, as a manufacturer, interbike was the performance date. working backwards you have certain target events. brochures printed for example. so, art to the printer by a certain time, so that the printer can finish the brochures in time for interbike. then backwards from that, photos to the graphic designer who’s preparing the brochure art. back from that, the photo shoot to generate the brochure photos. back from that, a finished next year’s bike to the photographer. if you work chronologically, you don’t take into consideration the time required to get these intermediate tasks finished. if you have a hard and fast target date, then as you work backward from it you find out how much time it takes to get to the starting line ready.
yes, injuries, illness, set you back. bummer. if they set you back too far, you skip the race. if you start your campaign too far in advance, you end up one of those guys who posts here, “it’s two weeks to the ironman and i have no desire to do it.” so, depending on the athlete, the campaign is 8 to 12 weeks. and that’s for the first IM of the year (if you’ll do two). that’s for, say, IM lake placid. then its recovery, working your way back to general fitness training, then getting ready for kona, that campaign is might be 6 weeks, or even as little as 8 weeks, and that includes another 3 week taper. (this, if the two ironman races are not too far apart from each other.)