Anna B reports her train this morning was two hours late:
... My train died at my stop and we wound up having to be pushed into Boston by another train. On top of that, the conductors didn't even announce the correct stops. ...
That's just over 9 miles, about 2 to 2.5 hours non-stop walking. Given the fits and starts for the train, you'd probably beat it slightly and arrive tired but more relaxed than sitting and stewing.
I'm at the far end of middle-aged and fairly stout, and generally manage about 3 to 3.5mph at a steady pace, so it would be a lot closer to 3 hours for me. In today's heat I don't think a 9-mile hike would leave me especially relaxed, unless you consider heat stroke relaxing.
I was waiting on that train at roslindale village and the sign said the train was going 5 mph. So that's an hour to South station. Fortunately the bus to forest Hills is right there but you can count on it like the sun rising in the East that the first heat wave and snow storm the train is gonna break down.