Spin Studios near Boston?

This probably doesn't help, since you don't want another membership, but I live in Arlington, too.  I belong to the Woburn Y, do the masters program there, and take their spin classes.  The masters swim is what you put into it.  Not as intense as some of the other programs, but still a good, structured workout.  The spin classes are hit and miss, like most.  There are some great instructors, and others that just have you mash for an hour.  They are, however, moving the spin bikes into their own room and beefing up the schedule to add more classes.

Maybe something to think about when it comes time to renew your memberships...

The Boston Athletic Club in South Boston has a good spin studio and a few instructors with cycling oriented classes. Mon & Fri Josh, an accomplished endurance  runner and triathlete, teaches a 1hr class at 6am. On Thursday, Terry, an accomplished triathlete and BAC's triathlon coach, teaches a great class at 6am, with some pool work after. Laura, an accomplished triathlete as well, teaches Wed 6am.

These 6am classes are well populated by triathletes, so you will have some like-minded company. And traffic is light that early.

You can get a free 1 week membership to try it out. I'll bet they would work out a plan rate for you to take just spin classes - no gym is turning away money these days. Also, they have loads of free parking, an attached restaurant open 7am to grab a bite/take-out and easy I90/I93 access.

Thanks for all the ideas.  I considered the Woburn YMCA, but since I share a car with my husband it probably wouldn't work.  Also, it's good to know that there are some spin classes around Boston, but I really want one that I don't need a monthly membership for.  I'm already signed up for the work gym for 2 years*, and masters swimming is ~$60 a month .

(You might think, why stay with the work gym, well, I was a part of the group that lobbied for a gym at work, and my membership -along with others- helped them buy equipment.  I'd look like a big hypocrite if I backed out now, and it is convenient)

Both Landry's in Boston and Fast Splits in Newton do classes through the winter (your bike on a trainer - landry's your trainer, Fast Splits their computrainers).

Great workouts, and on your bike instead of a spin bike with a fly wheel.