Gauge Your Fitness as a Curve
Anyone who's been active for an extended period of time has most likely noticed that their level of fitness at any given point in time is not a fixed variable, that is to say that just because you're at your peak fitness today doesn't mean that if you stop working out for three weeks you'll be able to jump in exactly where you left off. For this reason, it is most useful to think of your efforts as a fitness curve, and not a straight line. Your fitness curve may have a few bends in it, and your best today may not be your best tomorrow. But sometimes when you're forced to take a step backwards you end up taking two steps forward in the long run.
Any successful fitness regimen should allow you ample time for recovery. You won't always be pushing yourself forward, or going at your maximum effort. Your hard workouts should generally be followed by an easy day, or a day off altogether. If you're a more advanced athlete you may go for a couple of days at an increased level of effort, but either way every active person needs time to recover. You don't get stronger when you're working out, only when you're recovering from your workouts. In this way you are curving your fitness.
Unfortunately from time to time your fitness efforts may have to be reduced because of an injury. While this can be extremely frustrating, recognize that with proper rest and time to heal you can come back stronger than you were pre-injury. But viewing your fitness as a curve, you should also acknowledge that you shouldn't start up again exactly where you left off. Start back into your routine easily and give yourself several weeks of easy working out before you start ramping back up slowly. If you do too much too soon after an injury you're putting yourself at risk for re-injuring yourself, and over-training. Neither of these things is good for your fitness in the long run.
Viewing your efforts on a fitness curve can be very liberating. Knowing that you're always doing your best will allow you to always enjoy your fitness activities, if you realize that your best today may not be your best tomorrow. If you're tired today you'll need to redefine your concept of what your best is today. Your fitness curve mentality will keep you in the moment, and let you focus on what you need to right now to maintain, or get back, your peak fitness.