I don't set mileage goals. For one thing, I don't have a way to measure mileage since I decided not to use a computer any more, for another, I have other activities that I do (swimming, cross-country skiing, a little running) that are just as important to me as cycling. I aim to get at least 6 hours of aerobic exercise per week as a base, then focus on increasing a particular activity if I have a plan for it. I'd love to do another Swimtrek tour, for example.
For this coming winter I'm on the fence about riding in or not. On the one hand, it's good for bike handling skills, on the other it isn't really enough to qualify as very much exercise; I have to do a lot of planning and clothes packing for just 20 minutes or less of actual riding each way. I can get a higher quality workout in on the turbo trainer. If I can figure out a way to carry skis with me, maybe I can ride to a park with ski tracks and get another workout in that way.
I was actually thinking of running to work 3 times a week between November and March (well, running with a lot of walking breaks). It would take at least 30 minutes and be much more intense than riding.