One of the big things that I have noticed over the years, not only with my own fitness, but with many others is that too many people allow themselves to detrain too much in the off season. They spend the whole summer swimming, biking and running and then let themselves get completely out of shape in the winter. Because of this they never progress. The idea is to take some time off but not so much that you regress back to beginning of the season fitness. You want to maintain some of that hard earned summer fitness so you can build on it in subsequent years.
Switch gears and cut back on the volume but maintain some of that intensity. One of my favorites is cyclocross. It is done on a cyclocross bike which is a offroad version of a road bike with cantilever brakes and skinny knobby tires. It can also be done on a mountain bike. Courses are usually on grass in parks. Races last from 30 min to an hour depending on skill level and are very intense. Cross workouts are also fun this time of year. Lay out your own course and do laps. You can do one lap hard, one lap easy, or mix it up with short intervals and sprints. Since it is offroad, pace is slower which makes it easier to stay warm. Also the offroad aspect with its unpredictable traction and cornering helps greatly with your bike handling.
The same philosophy applies for running as well. Cut back on the miles but still run fast once in a while. Do some local 5ks and, and if you can find them, some mile races on the indoor track. Work on your form, maintaining your chest high, with a good arm swing and high knees.
For the swim, work on maintaining intensity as well and spend time on form work.
Weight training and flexibility are other areas that you should focus on during the winter. There are three components of physical fitness, endurance, strength, and flexibility. The last two are neglected the most because, lets face it, they are boring. As you age though, they become more and more important. I have noticed that my 41 year old body is more prone to injury due to strength and flexibility imbalances than it was at 21. The thing with strength and flexibility is that you "lose it if you don't use it" so make sure to become consistent with them, especially if you are over the age of 35. So get to the gym, fine tune you weight program, and sign up for the yoga class.
Remember it doesn't take a lot to maintain that hard earned fitness over the winter. Just a little bit every day, some consistency and you with find yourself in every better shape than ever when the warm spring days come.
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