Bill, do some long dayhikes with a heavier pack and see what you can do. It's pretty much an individual matter. Plan easier trips early in the year and gradually increase your mileage, speed and pack weight.

I have been slowing down the last few years, unfortunately. For me, I figure 1.0 miles per hour average, including a 10 minute break out of each hour. One reason is that i've gotten quite slow on both steep downhill and rough spots. Trekking poles help, and definitely speed me up on uphill and level trail, but as soon as I hit a rough spot or steep downhill, I have to pick my way slowly. I actually go faster uphill than downhill! For backpacking planning, I figure 5-6 miles per day. It gets better later in the trip as the pack gets lighter!

Several issues have slowed me down. One, I had to take almost a year off due to severe plantar fasciitis (never neglect Achilles tendon stretches!). I still haven't quite recovered the fitness I had before, such as it was. Second, because of a detached retina in one eye some years ago which left scars, I don't have binocular vision closer than about 6 feet. It's still fine for driving, but a bit iffy for what's under my feet, (Threading a needle is really hard!) Third, my balance, like the old gray mare, ain't what it used to be, mostly because of that year of plantar fasciitis. I'm doing quite a few exercises to improve this, and they do help.

Not that I plan to stop backpacking, as long as I can manage to put one foot in front of the other! I just have to be more careful and lower my goals!