allright so:
1st: duels are quick 1v1 combats, healing clases are meant to use their distance, cc and casting speed to heal themselves, not just the LoS an object can provide.
2nd: duel zones are big enough for a hunter to be max range and still be in the duel zone, so no, duels zones are not too small
3rd: moving in a straight line is indeed the best way to gain distance, but this is not world pvp, this is a duel, you cant run forever, you need to turn back and fight eventually, the edge of the duel zone is meant for that
4th: the best place to be when attacking is at the target's back, and you wont achieve that running in a straight line, so yeah, running in circle does work.
5th: you dont need to start on the edge of the fighting zone, start close enough to do damage but not melee range (unless you are melee), after that, kite in a circle pattern, you will see it works much more than just "turning around and run"
i cant tell you how to properly use your class/spec during a duel, you will have to figure that out on your own, but if 99.99% of the server can duel with that size, you should be able to do so as well