Whereas Alton Bay was "the gateway to Lake Winnipesaukee", I put in Alton Bay in Google Map with a destination of 19 mile bay. I came up with 16.5 miles.
Back in the day, as they say, many would arrive at the lake via railroad, and I was thinking that perhaps travelling on the east side of the lake, via the old rte 28 and Kings Highway, and starting at the old railroad station on the west side of the bay by the Mt. Washington dock, it may very easily stretch out to 19 miles. And another mile or so to 20 mile bay.
Just a thought
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