I've done the marathon multiple times, my wife has spectated a couple. Her biggest advice would be to only try and see your runner once along the course. It's a point-to-point and they are likely speedy so unless you have a vehicle, it's too hard to see them multiple times.
We've never stayed near the finish line, just preferred to save $$ and stay a bit further out, public transit is good and I've always used it to access the start line for the busses. We've done Cambridge, North End, Winthrop, West End. Must-dos for us are Mike's, Parker Rolls at Omni Parker House, Yankee Lobster, and Harpoon Brewery. When we've had newcomers we've done the Duck Boat Tours too.