There are certain aspects of life we tend to describe as being "necessary evils," and quite often exercise is one of them. We know we need it, but we don't always wanna do it. If you can honestly say that you wake up every morning totally stoked to work out, then you're a freak and I applaud you! Even I have days where the thought of hitting the gym or going for a run makes me want to throw a fit—and I'm a personal trainer! I'm supposed to love everything about exercise. But just like you, there are times when the motivation to move simply doesn't exist.
So, what to do about it?
First off, I say make your excuses. Get it all out of your system, then remind yourself why you chose to become a runner. Or why you joined that Body Pump class. There is a part of you that loves it, and that's the part you need to harness when the going gets tough.
If you're physically exhausted, that's one thing. You'll be setting your body up for failure, which puts you at risk for injury. But if you're mentally exhausted—there's no better energizer that a good sweat session. Tell yourself it's how you can take your mind off of work or that growing to-do list. It's a length of time in your day that you can totally own for yourself and no one else. Plug your earbuds in and just zone out while you pound pavement, or tune into the instructor and feel the burn.
Every good burn, however punishing it may feel, is actually a reward for hard work. (Bad burns, on the other hand, are injuries. Just make sure you know the difference.)
And speaking of rewards, set goals and give yourself something when you achieve them. A new song from iTunes, some running socks or a pair of shorts. Whatever those rewards are, you deserve them and if you have something to work toward, it might make that work a little easier to bear.
Finally, one of the best ways to maintain motivation is to partner up with other fitness fanatics that have the same interests. Join a class (if you haven't already). Not going means so-and-so that always sets up her step next to you might not be able to share that juicy gossip. Or find a running buddy—they always know exactly what to say when you feel like quitting a mile away from the finish line.
Bottom line, motivation has an uncanny way of coming and going but you are a strong individual with great goals. Believe in yourself and those goals and you'll find a way to do just fine when the going gets tough.
Now get up and go, slacker!