Your XP is going to suffer severely because of the high level of the hunter. I tend to use high level characters to help a lowbie finish quests as quickly as possible since you do get full XP for quests of course. If you don't place your characters in a party and just put the high character on follow, you can play your lowbie normally and just use the power of the high character when you run into trouble. If the high character is a healer (and not partied with the lowbie) you can heal without losing XP for kills.

Basically you can power your way through instances for loot and through quests for quest XP. However as your lowbie gets closer in level to the high character you can start letting the lowbie "tag" mobs (get in the first hit) and let the high character finish it off. As long as the mob is not gray to the high character, the lowbie will get full XP for the kill.

With your high character at 70 you could start this as soon as you get into Outland at 58 but will have to take your characters into an area with 63 level mobs as I believe 62 and below will be gray to your 70.

You could actually start the "tagging" routine sooner than that if you have some alts at lower levels than 70. For instance if you had a 30 level character, you could help your lowbie get through his 20's.

I did this with a couple of Warlocks at level 30 helping a priest on my third account. At level 18 I took them to Tarren Mill and started killing the spiders, bears and whatnot in that area. The 18 would tag them and the 30's would take them down. It was sorta tough starting at 18 as I would often have to throw more than one spell by the priest to get a successful tag, but the warlocks would finish mobs off before they became a danger to my priest. Unfortunately I found out that using my pet demons would reduce the amount of XP for the kill, even though damage directly from the Warlocks would not.