The Railway Trail in outer Sandys Parish [1] is one of the nicer stretches to walk, with a flat shady trail passing farmlands, residential neighborhoods, forts, and dramatic seascapes. Get on at George’s Bay Road, at the former U.S. Naval Air Station Annex; the trail leads all the way to Mangrove Bay, with wonderful views of the Great Sound, and tributaries that allow you to break off and visit sights along the way, like Fort Scaur [2].
Be careful of locals on speeding motorbikes along the tarmac sections; while the trail is supposed to be off-limits to motorized traffic, residents are allowed access, but many take advantage of that loophole to break the speed limit while away from the main road.
Links:
[1] http://www.moon.com/destinations/bermuda/sandys-parish
[2] http://www.moon.com/destinations/bermuda/sandys-parish/around-sandys/sights/fort-scaur-and-park