I think that Disney GOT the idea of the Challenge from people doing this.
Unless something really dramatic happens, yes you can make it. You might not feel great, but you can do it. Even with my slow slow pace, being slow post-race, sitting and putting arnica on my feet before putting on flipflops, etc, then taking the bus back to AOA and walking all the way to my LM room, I was back, changed, packed (DH and DS had taken most of our stuff), and out of that room before checkout time for this year's January Half. If we had been heading to a cruise we would have gone faster with everything post-race and hit the road to PC! Might even have been able to take Disney bus transportation (they tend to meet after 11ish), just to make sure no issues on the road to the port would stop us.
The standard 5K medal is pretty awesome now. It's not the hard plastic Mickey ears anymore, not since this year's Challenge. So if you go and have your own challenge, you'll still have a great medal, and you won't have had to pay the extra $75 for another 5K.
The Challenge cruise leaves on Monday b/c the January even has the full marathon on Sunday, and that wouldn't be OK to have the cruise leave that day.