Nick, You describe my hiking pace and style exactly. I'm 43 and hope to keep it up at 61 like you. I agree that Jeff "should" not have a problem. Somethings that drive this are age and health and pack weight. Both look to be reasonable, but would think shaving 5 pounds off your back might be helpful.
Some considerations….
Does the 20 lb max truly include all gear, food and the most water you will carry on your back? Casual backapckers tend to carry more water than they often need to…Jeff how much water do you think you will carry when fully loaded. I am questioning this because you might end up with more weight than you think and lbs over 20 will reduce your chances.
Next concern is not legs holding up but is blisters. Do you feel comfortable that your shoes will not blister on mile 7? Severe blistering can be hard to manage.
Pace, this should not be a problem, except that this is a time of year when day light can be your enemy. I have found that you can cover 2-2.5 miles/hour at a leasurely pace (including breaks and sightseeing). A safe bet is 2.0. Now this means you either need 9 hours of hike time or you need to up your pace. My observation is typical backpackers hike for far less time than I do and sometimes try to make distance by hiking at a fast pace, especially if they are young and in shape. If this group is planning to hike for 6 hours (i.e. 11 am to 5:00 pm) and cover 18 miles I would suggest this will be hard for you and not enjoyable.
Often more important than distance is elevation gain (loss isnt much better). Elevation gain over short distances is the most difficult to deal with. My rough guideline is once getting over 3000 feet of elevation gain your are dealing with enough that may really slow you down. Even just 1000 feet over a short distance like a mile is hard and can slow you down to a snails pace.
So here is my thought. If you can really keep your pack weight down to <20 lbs total; if you feel good about not blistering; if you will be hiking for 9'ish hours to cover 18 miles; and if the elevation gain will be under 3000 feet over the 18 miles then my opinion is you should be fine.
Jamie
oops. I missed the post were he said he was going for it. It will be interesting to hear how it turned out.