A few things to consider...

First, the C&O isn't that long of a tour so the penalty won't be as great as if you were starting a multi-week multi-thousand mile tour.
Second, it may be fine from the start and not require any real breaking in.
Third, consider that most of the breaking in with saddles is really breaking in of your butt.

Definitely do a few rides to see how it feels before you go. If it feels bad at first, persevere a little to see if it improves as you adjust to it. Don't push to the point of getting too sore or getting saddle sores. I find that a lot of times a saddle that doesn't feel good at first winds up fine after a few rides. This is true for saddles other than just leather ones IME. For example I hated the saddle that came on my Windsor touring. I rode it a few hundred miles, decided it was okay and used it on the Trans America. It was fine and I did many long ride on it including other long tours.

Experiment with saddle position. It is pretty critical with leather saddles. A lot of folks find they want the B17 slightly nose up to prevent sliding foreward.

I am not a Brooks fan, but my experience with my one and only B17 was that it was just OK when new and newish and by the time it was really well broken in I hated it. Given that, I'd caution against doing things that speed break in much including letting it get soaked or riding when it is soaked if possible. Using Proofide as directed and nothing else is probably wise.