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! :)
The cold itself isn't going to hurt the horse. Just warm up slowly and stay off your horses back until he's warmed up. Then when you're done make sure you take the time to cool off your horse properly.
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.