I've been commuting 12 months per year in MA, so it is possible to ride year-round here. I don't ride with significant snow on the roads or if the roads are substantially narrower because of large snowbanks, but light snow and ice are not a problem (I have studded snow tires). Cardio-type workouts at the gym (or home on a trainer) are probably useful as a substitute for real riding, but I don't do any of that. I also find that doing squats and deadlifts with relatively light weights and high repetitions helps my cycling. I do those at home, as free weights are inexpensive and don't take up a lot of space when I'm not using them.