Here are my 2 cents:
1. Yes to back-to-back training for mental purposes + a couple tips... When I trained for the Glass Slipper last year I did 5 back to back runs every other week and I did them at the same sort of "speed" I was planning to do the 10k. In my case, that meant I did not walk the 10k and run the half. Walking and running does use different muscles and I didn't want to mess up my legs for the half by rapidly walking 6 miles the day before. I did the back-to-back training mainly because I had literally NEVER run two days in a row before then. I was really nervous about pushing my body too much so I always had a rest day in between. For me the back-to-back training for GSC was a good opportunity for me to see that I actually could do it! I wouldn't recommend it unless you have a pretty good base though - so start early building base mileage. And on that note - make sure you plot out your weekly mileage to watch that the mileage doesn't jump too quickly due to adding a 4th run in there. keep the mileage build up slow and you should be good overall.
2. ICE BATH (at least for your feet) after the 10k. After the 10k, I ate, something, came back and iced my feet, took a nap, got a massage, had dinner, and went to bed. (In WDW you have to get up at a ridiculous hour. In DL it's not as bad.) That ice bath really really works!
3. Bring two pairs of shoes, but wear good shoes for the 10k also! I personally brought 3 pairs of shoes, probably all identical, I don't remember. I am a bit phobic about not having enough shoes because it took SO. Long. to find ones that fit well and worked for me. But if you see that it's not going to rain, wear your good shoes for the 10k also - last thing you want is bad shoes screwing you up for 13 miles the next day. I hear newspaper stuffed in shoes dries them out pretty well.
If I think of anything else, I'll pop back in. Have fun! It was a cool challenge and you can't beat extra bling!