I quite like going for a ride on those crisp sunny winter days so long as the wind isn't too strong. However if the temp is at or near freezing, I will always ride either late morning or early afternoon, so the sun has been up for a while and had some time to warm up the ground. I never ride early in the morning or late in the evening if there's any risk of ice, and I'm especially wary of shaded roads with tree cover or building shadows. I don't have to ride, I've got legs I can use instead if the conditions are too dodgy for safe cycling. Walking is actually more pleasant than riding when it's freezing anyway, because you're moving slower and don't get the same cold air blast in the face. It's all activity at the end of the day, even if not intense. Most people are going to be on the sofa all day in winter, so even low-intensity activity is more than the majority will be doing.