Hunt has a nice advantage coming into this fight, which is that his last fight was also a five-round affair. Not only was it scheduled for five rounds, but it also went the full five, and Hunt won the fifth. He understands the cardio necessary to go the distance, and Nelson frequently doesn't have it.
Unfortunately, Hunt has been out due to injury for awhile, and by the time he steps into the cage, it will have been nine-and-a-half months since his last fight. Hopefully ring rust will not be an issue.
Roy has only been scheduled for five rounds one time, and the fight only lasted three minutes. Nelson has never had to go more than 15 minutes, and it could be an issue for him. In addition, in Roy's UFC career, if he can't knock out his opponent, he loses—guaranteed. Knocking out Mark Hunt is a tall order. It's only been done twice in MMA, and one time was by Junior dos Santos.
Another intangible for Nelson is whether or not he chooses to use his grappling. If Nelson wants to take the fight down and submit Hunt, he has that potential. However, Nelson generally chooses not to grapple, and it would disappoint a lot of fans if that's how he wants to win this fight.