Mental prepping for a rainy day

I was scheduled for a 6mi run for yesterday after almost a week off. During the day, in between things I would imagine what the run would be like. For no apparent reason, I kept thinking "3 miles", when I've been incresing to 6 miles lately.
I don't know why. Maybe it had to do with the fact that I'm not a natural runner, maybe it was the weather (cloudy with rain expected). For some reason I could not bother thinking past 3 miles.

So I get to my course after work as usual and I feel nice and strong for the run. My polar indicates I'm doing about an 8min/mi and I totally believe it (that's fast for me). After about 1.5 miles I'm not feeling as strong so I ease the pace to around 9min/mi. and really start feeling the effort kick in at around mile 3. I take note of how I feel physically and do feel the straing (maybe I took too many days off from running?). I tried convincing myself that it's just mental but my HR was quickly going up to around 165 (not to comfortable by this time).
So no happily, I decide to cut it short and get some rest in prep for next time.

By the time I get to half a block from home base I noticed my distance was about 4.3mi but now I really want to do at least 4.5mi. So I take a route I never went on and it takes me on an up hill. I keep going another block and my HR is now reading around 173 at this point BUT! my pace is 8.2min/mi and my distance is now 4.8 GOING UP HILL! (HOW COULD I NOT GO FOR 5 MILES NOW?? I'LL KICK MYSELF TOMORROW IF I DON'T).
By this time it's raining pretty good and I need to zip down my rain jacket (THIS FEELS AWESOME). I continue like this to finally succumb at 5.72miles. I feel the satisfaction of going hard, finishing strong and going past what I thought I could do for today. I felt like jumping up and down to celebrate.........but there were too many people around.
Today, as I sit here pumped from yesterday's run, with those satisfying aches that tell you, you've done good, I mentally kick myself for not going to mile 6. (I'll put this on the mental shelf and use it as motivation for next time).

My point here is now matter how well you prepare physically, your race or work out is done long before you even change to your work out clothes if you are not prepared mentally. Think positive, remember your good work outs and the feeling you get/got from it during and post.
If you're having a bad work out, pack it up and don't worry about it (your body and/or mind may need it more than you know). It's called a bad day for a reason. If you continue pushing it while having a bad work out you'll spend needless and counter-prductive energy that will serve as de-motivation.
If it changes for the better like my work out did in yesterday's run, then by all means, knock your self out, make it a good strong finish. Remember that awesome feeling and use that to motivate yourself for next time. I call it progress.