Buy a Boston travel guide and look at the included street level maps. It’s basically Downtown Boston, ranging from the southern part of Charlestown and the northern part of the south end in the east, and the Fenway area in the west. They will also usually have a breakout map for Harvard Square and maybe MIT.

This is basically the non-resident‘s version of Boston, and there are essentially no two points on that map that are more than an hour walk from one another. You don’t even need mass transit, most stuff will be less than a 20 minute walk away by foot.

If you want to go to Costco, some fancy restaurant in Newton, or go pyo apple picking, then walking isn’t very realistic.

It might not always be walk-friendly, but due to the distances involved, the core is walk-able.