Pat in DC, You are very likely younger than I am, (closing in on 75 years,) but my travel companion for this trip is also younger by a decade and a much stronger rider than I. We both were glad we settled on 50 miles a day and tried our best to schedule the camps accordingly. After more than a decade of doing rail trail multi-day trips, three things I have found feels proper for me:
1. A 5-minute break every 10 miles or so. Mainly to relive the butt and resupply the tank with a small snack. (I rarely stop for a "real" lunch unless it is some kind of coffee shop stop.)
2. 50 miles a day, riding loaded seems to prevent being too weary and making the trip a chore. A little time in camp, even if just a relaxed meal preparation is nice too.
3. No matter how fast I can ride if I have too, like making up lost time, beating an incoming storm, etc., It's hard and unpleasant for me to sustain 12 - 14 mph all day. I seem to always fall back to 10 mph, unless riding surfaces, etc. are perfectly smooth and level.
However, your schedule is your schedule and if it is inflexible, you may not have a choice. Depending on your exact start and finish locations, the GAPCO is pretty close to 350 miles (50 miles x 7 days).
Paul Germain
Midlothian, Va.