One of the most overlooked elements that needs to be addressed when training, or developing a training plan is an individuals training load. Training load is made up of these things; training, stress, and everyday physical demands. Stress and everyday physical demands play a huge role in training load as well as estimating how much recovery you may need. Let me give you a quick example of training load influences outside of actually training.
You wake up to 5" inches of wet sloppy snow before heading into work. So you shovel the snow, shower, eat, and rush around like crazy just in time to sit in traffic for an extra 40 minutes because of an accident. Your have now just added an extra 35 minutes of physical exercise, as well as a ton of stress from sitting in traffic. Both of these things take a toll on your body, one physically and one mentally, and they need to be treated as such. These things need to be taken into consideration when planning your training for that day or even week. Physical work performed throughout the entire day no matter how small or big is still work, and stress is always going to have negative effects on our body, mood, and behavior. Exercise is a great way to deal with stress and to "burn off steam", just keep in mind that everyday you have an allowance of energy and need to balance that energy expenditure with adequate recovery. An occasional day of increased physical and mental stress might not have a great impact on your training but if they keep occurring it will continually add to your training load and decrease how much your body can handle.
Don't forget that participating in fun exercises, such as skiing, snowboarding, etc still use energy and place demands on your muscle. Sometimes people think that fun doesn't mean exercise.
What is the biggest reason to keep track of your training load? It will help to keep you from over training. Training correctly is about loading the body with work, and then resting it. While the body is resting it is recovering and growing allowing us to become faster and stronger.