"His friends, experienced long distance thru-hikers, say that the JMT is a real kicker and was very difficult in the three weeks they did it in last year in August/September averaging 12mi/day and suggest we take 28 days ourselves. I have no idea if they are lightweight or heavy haulers"
Ryan, just to give you another perspective on it. My girlfriend and I did it in 21 days (18 days hiking, 3 zero days) last September. We're both of average fitness, but we literally spent the majority of the prior 3 months before our trip sitting at a desk studying for the Bar Exam...pretty much zero active movement. After taking our exams, we had 3 weeks to shape up, gear up, dehydrate food and post our food drops before our hike started. We both live in Boston, so like you, we had zero elevation advantage. Everything turned out fine.
We became gym rats for those 3 weeks to rebuild some foundation of fitness. But what was key was really focusing on keeping our base weights low. Mine was roughly 12lbs with bear can, hers was 10. We used every food drop to our advantage, carrying as little food in between resupplies. So yes, we used TM, Reds, VVR, and MTR.
Dare I say those uber-experienced thru-hikers were wrong? I would say 3 weeks is a very leisurely pace. We would have taken less days (and zero days) had it not rained so much this past year. I think the absolute hardest part is just getting out of the valley to TM. I think we were only doing 7-9 miles those first few days. By the end of the first week, we were comfortable doing 12-15 mile days. Often we would roll into camp by 4pm, choosing to lounge around and be lazy as opposed to hiking til dusk. Also, we stopped a lot so that my girlfriend could take photos. So by all means, 3 weeks (or 18 days for that matter) is very very doable.