AMC formula -- Younger fit folks in the area consider the Appie formula ("guidebook time") pretty conservative. I guess it has to be since it is used in their published guidebooks, and believed by the general hiking public.
"older, inclined-to-birdwatch crowd" -- one interesting thing in the local Sierra Club is that there are two day hiking sections. The one populated mainly by post-retirees does day hikes that are more than twice as long (20+ miles) as those in the other (younger) section (8-10 miles). What struck me most was not the separation into the two groups, but rather that the one that covered the most distance is almost exclusively the older folks. I would have expected fit younger folks there as well.
Equivalent miles -- I had not thought of it that way before seeing your post, but I like the concept. For one thing, anyone can use it for their own trips by applying their own base hiking speed.
Soreness -- the issue is DOMS, due to the eccentric contractions when going downhill. Suitable gym work can minimize that, if so inclined. Since it is a delayed effect I would not have expected it to affect time/speed that day -- just getting started for the next day :)
FWIW: I got to wondering about combining "equivalent miles" (eMiles) with calorie expenditure, based on some bits and pieces from another thread currently in progress on that subject.
*) It has a pointer to a calorie calculator that seems like a pretty good fit with several folks' hiking/backpacking experience, so I thought I would take that as a reference point.
*) Some folks believe that calories per mile gives a good estimate. It seems to me that you have to take elevation change into account, so I wondered whether applying it to eMiles might not work better.
*) The claim is that, other things being equal (terrain, distance, etc) speed does not matter enough to worry about -- but time does. I wondered whether using eMiles might not be a good way to extend that thought to varying ups and downs.
I tried it (a little) using three different ways of calculating eMiles:
1) Your way, as presented
2) Your way, but adding for ascent (only) and ignoring descent
3) Equivalent to the AMC way: trail miles + 1 mile for every thousand feet ascent
What I found, based on limited experimenting, was that method #2 came the closest. I do not know what, if any, relationship this has to reality but it was an interesting thought exercise.