... your left knee is wonky ... your legs are achy because you didn't taper enough the night before ... your stomach is telling you it either didn't like last night's food or there wasn't enough ... you forgot your lucky socks ...
... and you still have a BIG RACE TO RUN!
During the week I do inclined treadmill fast walking, elliptical and bike. Every Saturday morning, I drive to the gym (Michigan, snow, you get the picture) to TEST MYSELF with a weekly run at a 15-min mile pace. And every Saturday I add a lap to the run (.25 mile). So each week it's a tougher challenge than the week before.
I get that "can I do it", "did I eat right" feeling on my way to the gym. And while I'm running, I really can't stop -- it's my weekly test. Other days of the week, what I set out to do and what I actually do may vary. But not on my Saturday race day.
Today, by the way, was my 4.5 mile day. And because of the items I mentioned above, it was a tough run. But the three times I felt like I wanted to stop, I just had to say "it's race day, just keep moving forward"! I also looked at the WDW race map (print from online) before I started, and then as I hit each mile, I visualized the location I reached (race track, Poly, MK arch, so far).
And I kept running until I had done 4.5 miles. And that's the longest I've ever run in my life without stopping ... until next week?
If you're training for the Half Marathon, consider establishing a weekly "race day." Pick a day and time of the week. Then decide on a starting point. (Mine was .50 mile, 2 laps, at speed 4.0, which is a 15-min mile pace. But you'll want to pick a reasonable starting point based on your training so far and your running ability.) Then each week, add a lap and keep at race pace or faster, and keep running! It really is good practice at dealing with both the physical and MENTAL challenges we'll be faced with.
I think you'll really benefit from RUNNING A RACE each week!