Steve B makes a good point about the bike trail surface - regardless of the various interpretations of the labels of the surfaces
the rolling resistance is higher on the smooth crushed limestone bike trails - compared to paved trails and roads
Steve’s point is mostly irrelevant to the OP and gives the impression that the paths are more difficult than they are.
The OP does brevets, which entail riding 125 to 250 miles in a day.
As far as “gravel” goes (or even not), the GAP is an easy surface to ride on. Some people do it on road bikes. The C&O surface isn’t difficult either but it’s narrower and worse in rain than the GAP
And they are also flat as hell (*). That is an aspect of the “difficulty” people are just passing by. The GAP does have a long uphill but it’s very gradual. It also has a long downhill.
The OP should have no issue at all doing 70 mile days on these.
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It appears that some people:
- aren’t aware of what this subforum is about.
- haven’t done the sort of riding this subforum is about.
- are not considering the experience/conditioning needed to do those rides.
- have never actually have ridden on the GAP and C&O.
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* there are a few places were one has to go off the GAP onto roads. Those can be fairly hilly but the distances aren’t very long.