Some thoughts:
#1: Have the group run into an advance scout for the gnolls. If they speak the language, he can try to escape and get the word back to the larger band he's scouting for. Similar ideas can let them know they're being followed, and by a large group. A cautious group is not going to want to fight a group that outnumbers them by 3-4x, and is likely to flee.
#2: One way to make the flight interesting is to allow them to set up traps and diversions as they go, to reduce the numbers chasing them. If you're going to want them to turn and fight the hunters at some point, this is a good way to set it up so that flight is believable to start and eventually they can set up an ambush for the hunter. Depending on your group, they may love the idea of using the flight to whittle down an impossible odds fight into something they can manage, and with so many trained in Survival you can bring it up as a possible tactic they'd know with that skill.
#3: A good place for an ambush is a great opportunity to handle their chasers, so you could go with the idea of having them fight the hunter at this point, after they've demoralized his band (who have now gone several days without catching their prey and are likely ready to get back to normal plundering) and reduced the numbers.
Good luck! It's tricky to figure out how a group will react to this type of situation, but it can be a lot of fun to run as the flight tends to have a lot of roleplaying options. (As long as they don't have pass without trace/etc, where it becomes considerably more difficult. Consider using a druid hunter in that case, or use animal companions to help track).
