In that case, your party members would gain 20HP. This is NOT a one time effect to my knowledge - they will continue to benefit from it as they level up.
The "effect of similar abilities" text is to signify that later Health Level bonuses replace the earlier ones, rather than stack.
Unfortunately, as you've noted, you can only take on a single hunt at any given time. This drastically slows down the process of raising your Hunter Rank.
To make matters even more difficult, based on posts in threads like this one, it looks like Hunter stars are a finite reward and completing every hunt in the game will land you at level 10 exactly.
I've done practically all the bounties, and I cant find anymore to do. Im rank 9. Im 28/43. Am I suppose to do something to unlock more hunter quest? Ive done all the dungeons, done the story, even the pitoss dungeon. What else is there for me to do to reach LVL 10
It's possible that you missed a location for its hunts, I suggest having a checklist of each outpost / town and going to all of them just to confirm you did every hunt at each, I did similar things in games where I thought I did everything, but I just missed one area because the map showed me as having it available so I just assumed I went there.
Don't forget the newly unlocked rank 9 hunts at Hunter HQ. Iirc, they all add up to 15 points. Could very well be what you're missing.
I summed the Stars column in this chart and the result was 293 stars.
From this list of Hunter Rank requirements, the requirements for each level are:
5, 15, 30, 50, 40, 40, 40, 30, 43
These add up to 293, as well. Thus, in order to reach Hunter Rank 10, you will have to complete every Hunting quest in the game.
The most optimal method for leveling up your Hunting Rank is, unfortunately, to do every hunt as you encounter it. If you only do multiple-star-reward hunts, you'll end up having to backtrack and find all of the one-star-reward hunts you skipped. If you're doing the last hunt at a specific location, it might also speed things up marginally if you just go to the next tipster, since you can turn in hunts to any tipster.
Best Answer
About 315 miles or so.
Method: Assuming Shawn Gordon's source is correct and you will have to walk for 30 hours to get to level 10, we can use that to figure out how far you have to walk. All we need is to figure out how fast Noctis walks and then to do a bit of math.
I parked my car at Coernix Station and then ran SW down the road so that I'd have a straight path free of obstacles to walk. Parking my car at the station gave me a marker with a distance on it. I walked until just after the marker ticked up to .35 miles, so that it wouldn't actually be .359 miles or something like that. (I'm assuming it rounds down.) I walked as straight and steadily as I could to the marker, and it took me almost exactly two minutes.
Multiplying .35 miles per 2 minutes against 1800 minutes in 30 hours gets us about 315 miles. It might be a bit more than this if I didn't walk perfectly straight, but it should be pretty close.