When it comes to exercise, and really life in general, your excuses don't have to be "good", or even rational. They just have to sound better at that time than whatever it is you don't want to do.
As I currently work my butt off trying to get in half-decent shape, I can't help but reflect on some of the really good excuses I came up with during my recent deployment, which "helped" me get this way in the first place.
The mother of all excuses was my hurt back. On the surface, it seems perfectly logical to take a "rest" from exercise in hopes that it would help make things better. But the reality was that I was just as sore after taking months of "rest" as I had been when I was at least attempting to workout from time to time.
Another really good one was my eating schedule. If it turned out that I was really hungry after I got off work, and had to stop by the DFAC on the way home, well that pretty much ruled out the gym. I mean really, I might puke or something if I try to workout on a full stomach.
Speaking of the gym, they had a cleaning schedule that didn't always mesh well with my off time. And again, if the gym's closed, I couldn't possibly find somewhere else to exercise that day. Going to the gym was my "plan".
And then finally one excuse that really hit me when I got back home ... the showers! I have found it so much easier to work out twice a day since I've been home, simply because I know I can walk into my bathroom and jump in my shower afterwards. The showers at Camp Marmal in Afghanistan were horrible. Even if I wasn't hot and sweaty, I hated to go in there. So as you can imagine, not wanting to get hot and sweaty and having to shower before bed turned into a really convenient and consistent excuse to tell myself "I'll go to the gym tomorrow".
You know what though, if working out had been important enough to me, none of these excuses (or the 1000's of others I could easily come up with) would have mattered. I would have found a way to do it.
And that's really the point ...
If you find yourself not doing something that you think you should be doing, it's probably not the fault of whatever rationalization you've managed to come up with.
It just wasn't important enough (to you) at the time ... simple as that.