Cross-Country Skiing

To get an aerobic workout and see the woods from a different perspective, try Cross-Country Skiing.
Cross-Country skiing, aka Nordic Skiing, is a sport that takes coordination and stamina.  Don't let that scare you; you can develop both and find that your winter outdoor experience goes way beyond what you'd encounter on a normal hike.  The trick to it is learning to push off and glide along the snow, making long strides to cover a lot of distance in a relatively short time.  If you really work at it, you will develop a solid technique on your own, but a lesson from someone who is experienced will help at first.  The skis, boots and poles are different from those used in downhill (alpine) skiing.  Skis are long and thin with a texture underneath to help grip the snow.  The boots are flexible and ankle-high or slightly higher.  The poles are long and have a large bail at the bottom.  You don't need to spend a fortune for this equipment, but you might want to rent a few times before buying.
Clothing for this sport is critical as you will want to start off with less than you would for a hike at the same temperature.  As you ski, you will build up quite a bit of heat and will want to remove a layer or two to moderate the temperature.
There are a number of areas that create tracks for you and charge a modest fee to use them. Eldora Nordic Center is a good nearby place to learn and to find quite a large area of packed trails.  You can rent all the equipment and ski all day for under $40.  You can also go to Brainard Lake, near Ward, CO which has trails at zero cost, but you need to rent equipment somewhere else.  You can also ski in Rocky Mountain National Park for free.