Regarding the parking question:
I know of two options below and neither are flexible or ideal. I'd like to hear other options.
I haven't been able to make this official NPS parking lot work, because the parking lot used to close outside business hours. I'm not sure if that's changed, and I'm not sure if coronavirus has closed this lot.
Here's the NPS option: There's overnight AT parking available at the NPS visitor center lot. There's a permit system, they allow up to 14 days, and they'll shuttle folks to the lower part of town (though that schedule is limited too and you can just walk anyway).
https://www.nps.gov/hafe/planyourvisit/parking.htm
I've left my car at the Hostel International at Sandy Springs, Maryland (just across the bridge from Harpers Ferry). A stay at the hostel is required to do that. I've stayed at the hostel before and after hiking, to give myself an extra day to sightsee at Harpers Ferry (cool place, loads of interesting history, shops, etc...)
Problem there is the pedestrian bridge linking MD & Harpers Ferry is closed, and a hostel stay is def. not in the plans for most people.
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Wow, averaging 20 miles/day is fast hiking, especially for sections (as opposed to thru-hiking)! More power to you to be able to plan that kind of pace in advance...
Enjoy your time out there...