Yeah, it's not that I don't like riding outside in the winter, I'm just worried that it would be too cold to safely ride outside. Know what I mean? And thank you! :)
Footing, on the other hand, is a different story. I understand if you don't want to ride because the ground is frozen and hard, or wet and slippery. When it's frozen, just do light work. It wont hurt them, but I wouldn't jump or barrel race in it. And if it's slippery, well, basically the same thing. Just go slow and don't ask for too much. You can always go on a trail ride/hack too. Find some hills you can walk up and down to work the muscles a little more.
Sometimes in the winter when the weather is really bad, I just take my horse to a big fenced in arena and let her run. She gets her bucks out and burns a few calories, and it doesn't very long.
Also, I don't think they'll get too out of shape if you give them a month off when the weather is at it's worst. They'll probably even enjoy the break.