I don't bicycle during the winter, for a variety of reasons: in particular, it's a tad less safe, and I just plain don't enjoy it much when the temp falls below 50. And by the time we get around to spring, there is a very strong, "It's too much haaaassle" response -- in particular, trying to remember how to reinflate those verdammt performance tires. (I never remember the eensy-weensy little screw to open the valve, and wind up struggling with it for ten minutes until I get the clue.)
But that first good bike ride of the season -- a day like today, when it is clear and crisp, warm enough to be pleasant but not yet hot, going for a short 7-mile loop exploring a bike path I've never tried before (the one along Alewife Brook), seeing the sights and the people -- more than worth it. I really need to remember that going for a ride is a *totally* appropriate way to get my daily exercise, and I'm allowed to do it simply because it's fun.
And yay for the state getting it right: they have finally committed the funding to complete the Community Path over the next five years. This will effectively run the Minuteman Path (which runs into the Somerville Community Path) all the way to Lechmere, so there will be a more or less continuous bike path from Boston out to the suburbs...
(Back in high school, I typically did about 15 miles a day, but that was country back roads. I can't easily open up quite that much around here, even if I was in good condition.)
Anyway, if you can cope with getting out to Davis, that's highly recommended. The path essentially goes through Davis Square Station -- it ends at one entrance, you have to walk to the other entrance, and then the path picks up again -- so you can simply take the path as far out as you like from that point. Much of it is straight, most of it is well-paved, and the pedestrian traffic *mostly* isn't too bad, especially as you get further out the Minuteman...
I miss being able to rack my bike onto my car. That opened up all kinds of places the T makes inconvenient. The hybrid Camry's tolerances are too tight for my trunk-mounted rack to latch onto, and I haven't cared enough to get a trailer hitch installed for hitch-mounted rack, since that would affect my gas mileage all the time, not just when I had a bike on. I think my best T-assisted access to Porter-Davis-Alewife is probably Orange Line into North Station, then commuter rail back out to Porter. I wonder how hard it would be to move a bike through Charles? I know that is walking distance from North Station.
North Station provides easy access to the Charles River bike paths (via Nashua Street). You can ride out to Waltham and back, much of it on either side of the river, and it's quite a nice ride (pavement is better on the Boston side than the Cambridge side, downriver from Arsenal bridge). If you did want to run your bike through Charles, it shouldn't be any problem. The station was redone in the last few years, and there are ascension rooms from street level to the platforms. You just have to be concerned about the rush hour times that the T doesn't want you to bring a bike into their carriages.