Verified answer

Unless you take your horse in a boat from the North End to Charlestown, you'd have to make a wide detour west to cross the Charles and the Mystic Rivers.

Use these links to plot the route:

Google Maps Pedometer. This lets you track your proposed route, and shows the mileage.

Paul Revere's Ride -- map the Dawes route. Revere crossed over to Charlestown by boat and got his horse there. Dawes road overland. Note the route goes down the narrow Boston Neck, crosses the Charles into Cambridge (Harvard Square), then up to Arlington. http://nationallancers.org/routes.html

This map shows Colonial MA. There's a gray line about halfway between Menotomy (now Arlington) and Malden, that's a bit north of Harvard Square. Take that line to Malden to Lynn to Salem, and from Lynn to Marblehead. Plot Dawes Ride, then that gray line to Salem, backtrack to Lynn, then M'head, then retrace. http://www.sonofthesouth.net/revolutionary-war/map...

I get a rough 60 miles round trip.

Until about 1820 Boston was on a peninsula. To actually do that journey one would need to go south, then west quite away before turning north and ultimately east towards Marblehead.

The best map of the area in 1775 is the one depicting Paul Revere's ride - http://www.revolutionary-war-and-beyond.com/image-...

I'd suggest that you need attempt to find an old map from that period that has a legend (i.e. x inches to the mile) on it, then measure straight lines between the points that allow you to follow that path.

Just to make things more complicated, which definition of a "mile" are you using? There's been more than one..

was he using a bike from http://www.2wheelbikes.com/