Mousekedude. A good run is one you finish. Forget the time.
There will be lots of people walking and I doubt the sweepers are all that enthusiastic on a 5k as long as you keep moving.
Plod onwards, run/jog/walk as necessary to stay ahead of them.
If you are not ready by now training hard at this point isn't going to help any. In fact quite the opposite.
For half and full marathons you should be tapering down the training I guess the same is true for a 5k.
Go walk a mile, see how it goes. You will probably be just fine. Check you can do it in 16 minutes or so. (Pace required for the Disney runs.) If you can finish a mile in good shape now, you will finish the 5k.
Nerves are good. All those endorphins and adrenalin, thats going to help right
5k is 3.1 miles, you probably walk further than that on an average day at the parks anyway.
You CAN do this. BELIEVE.
I have also been suffering with breathing problems this year so I sympathies fully. Since I returned from my Goofy trip I have really struggled with any training. (This getting old thing really sucks
) I still managed to complete a half in September. OK a full hour slower than last year, but I completed it. (Finish = good remember
)
I tell myself that I'm starting to get ready for another Goofy. 2014 perhaps. or 15, 16 whenever..
Edit: You don't have to throw stuff away
It looks like the weather will be OK so carry the black bin liner in case of showers. (remember to cut holes for head and arms BEFORE you leave the house
) Throwing bin liners away isn't a problem.
I wear a sweatshirt on some training runs and tie it around my waist if it gets a little warm. No real issues.
"We live in a society absolutely dependent on science and technology and yet have cleverly arranged things so that almost no one understands science and technology. " Carl Sagan
Yea you all know the tune........ 
