We ride year round here in Alaska. We both have fat tire singles and built a fat tire tandem last winter (like Colotandem). I commute to work on a studded tire fat bike down to 0 degrees you just have to dress properly. We don't necessarily get fitter in the winter just try to maintain. We also do some trainer work on Elite brand trainers that are computer controlled with many downloadable rides on a big screen tv in front of trainers, not our favorite but sometimes the only way to get a ride in. Age is a number I'm a young 61 and we try our best to keep up with the youngsters on big rides and occasionally surprise them when we can hang. San Diego in the winter would be like summer riding here as far as temps are concerned.