You would know your market better on this one, but I think your concern is completely valid. On the one hand, once a week is what these people need. On the other, if they start flaking they might think they are not using it and cancel.

This is just me spit-balling, but what if you did something where you explained that people have very differing thresholds so you have created a weekly class for the self motivated, but you will only promote one class a month so as not to overwhelm them. So you send out several reminders the week of the big one, but tell everyone in each class that if they would like more, you do it once a week. But to avoid overwhelming anyone they need to set their own reminders. So you position it more like one is mandatory, and the rest are extra credit. You might word it a bit more gracefully, but you get the idea. That way you give everyone as much as they can handle, but don't create a situation where people feel like they are not getting what they paid for if they actually want a little less of a commitment. 

Just one idea.

Maybe, but I think many will select weekly because it sounds like more, but then potentially not do it and you lose them. Honestly, I would just pick one and go. You can course correct later if you see people dropping off. Maybe survey your list and ask what they would be more interested in?

Actually, I was sort of going against my own original advice along those lines. That would work, but it might be best to keep it simple and come up with one plan. Do whatever seems most common sense. then take the temperature after you have some customers in there and see what they want. Or more importantly, how they act. I personally like simple solutions. You can change it later.