I hope I didn't offend you with my comment, and I did include a smiley face at the end of the sentence.
What is my MPD on the JMT? Don't know. Haven't been on it since 1973, when it got too crowded for my taste. But, I am considering a High Sierra Trail trip in the future.
I do mostly cross country trips sans trails, and usually average over 2 mph. On some trips, progress is measured in hours per mile. :)
I will turn 59 this year, and am more active than most folks 30 years younger than me. I regularly run, go to the gym and hike. Plus at 5'11", 150 lbs I am pretty nimble. So, how I hike is not typical for most people our age.
Could I do better than 10 MPD on the JMT? Sure.
I can move over boulder fields and slopes much quicker than my college-aged son, who has run in the last 3 NCAA Divison 1 Cross Country Championships, because I am more confident and experienced. I can keep up with him on the flats too. This is a kid that runs up to 100 miles per week.
And for 11,000' passes... no problem, unless I get lost. :)
I can still do the Cactus to Clouds Trail in the San Jacinto Mtns in under 12 hours. This is a 22 mile trip that starts on the desert floor. It goes to the top of Mt San Jacinto with an elevation gain of over 10,000' and then comes back to the aerial tram station. In warmer months, I need to carry up to 2 gallons of water for this trip. I often do multi-day trips with a start at this trail which is a 8,000' gain in 11 miles, just to get to the San Jacinto wilderness. On these trips, 15 MPD is a piece of cake. I suspect that in the coming years, I may slow down some. :(