UHC was first created a group of YouTubers that I think altered certain settings in a server client. One of their fans decided to make a mod for Minecraft based on the idea these 4 people had. So yes, it is a mod. So I don't think (providing it used a separate world) that it would mess up the current ones we have. Also, yes they can take 10 hours but considering that the Mindcrackers record it for the entertainment of their viewers, we probably wouldn't take that long.
What could happen is if we say about an hour or maybe a bit more of getting ready (gathering resources and such) and then you have to go into the middle of the world to fight it out. You could however still attack people before the set time is up just to make it more entertaining and also it would be best that when the time is up people don't all go into the middle at once and just wander there. Having said that, it is preferable if you didn't take a couple hours to arrive as that would really drag it out. I think this way we could perhaps have multiple UHCs in the one day.
What could happen is if we say about an hour or maybe a bit more of getting ready (gathering resources and such) and then you have to go into the middle of the world to fight it out. You could however still attack people before the set time is up just to make it more entertaining and also it would be best that when the time is up people don't all go into the middle at once and just wander there. Having said that, it is preferable if you didn't take a couple hours to arrive as that would really drag it out. I think this way we could perhaps have multiple UHCs in the one day.