Well, EVENTUALLY it is bound to stop raining, if only for a few days here and there! Until that time you just have to do the best you can. As long as you are getting out for that long ride on the weekend and work up the mileage on that to 75 or so if you can, then you'll be fine. My short rides during the week probably aren't always as good a workout as a spin class would be, and I don't do them every day. You do need the outside handling practice, but you are at least getting that on your long ride and as the rain lets up, more often.