There are many days when I just don't feel like getting my exercise in. It's been a long stressful day full of cranky bosses and clients. It was a long stressful night full of cranky children and spouses. I'm just plain lazy. There are a dozen other things I think I would enjoy doing more.
On these days I tell myself that I only have to do a half workout or that I'll just walk. Yeah, it's an old trick but it works for me. Some days I really only do a half workout. But you know what it's half a workout more than I would have done otherwise. And more times than not, I actually end up doing a little more or a lot more than half a workout.
For example, last Monday was a combination of all 4 reason I wouldn't want to workout. I convinced myself to just go and walk. Then I convinced myself to do two half mile intervals of jogging. But during my second 1/2 mile I got absorbed in a magazine article (yes, I know it's weird that I read while jogging) and my legs and lungs felt really good. Before I even realized I had done 1.5 miles and managed to eke out another .3. And I actually didn't want to work out at all that day.
So, get thee to your exercise option. You never know, it might turn into your best workout in weeks.