A neat little analogy occurred to me the other day as I was finishing up a run.
Imagine chewing a piece of gum and blowing a bubble. I'm pretty sure we've all done this at some point.
Now imagine that bubble is your fitness.
Because of the porous nature of bubblegum, you have to keep adding a bit of air all the time in order to keep the bubble from deflating - just as you need to keep training in order to keep from losing fitness. As you slowly add breaths (or workouts), your fitness increases just as the bubble gets bigger. However, if you try to blow too much air into the bubble too quickly, you'll pop the bubble and have to start all over! If you overtrain, you run the risk of injury or burnout, both of which will require time off, loss of fitness, and a slow rebuild. You'll get better results from adding small, regular breaths than just trying to blow in big blasts of air at long intervals - consistency over time. Sometimes outside forces come along and burst your bubble, too - an injury, illness, highly stressful life event or even just a big project at work can act like a pin against the bubble you built with all your hard work, or just rob you of the gum you need to blow a big, strong bubble.
As the months and years of consistent training go by, you get to add more gum to make the bubble stronger, so it will hold more air without popping. That's building your base fitness. Then, if you know you have a race or big effort coming, you can let a the bubble deflate a little by adding less air so it becomes more resistant to popping - that's why we taper, reducing the volume of training (or breaths going into the bubble) before a taxing event. Your bubble needs to be solid enough to handle that race effort, over-distance training session or interval workout, or you'll stretch yourself too thin and quickly end up deflated.
So when you're planning your next block of training, make sure to keep those breaths coming - regular workouts and appropriate recovery will see your fitness grow, and if you feel yourself getting close to the bursting point take the time to back off before that bubble pops! If it happens, though, remember you've still got the gum; just start back at it with smaller, regular workouts, and soon you'll have a bubble of which you can be proud!