I know the OP stated no co op, BUT.. my buddy had me open a lvl 1 t-6 bouty run and he simply brought his lvl 70 in and mopped up the places, two bounty caches and I was at 69.Don't have to have a OMG lvl 70 character to drag you through, it's fine.
But on your own, doing the bounties for bonus exp CAN lvl you quickly, just no where near as quickly.
Even running rifts at low levels can raise you quickly.
And I wold add, unless you have a stash of lvl 70 gear waiting on you, it sux to be at lvl 70 and THEN have to go get some gear.
I started RoS with only a Wizard, then I got it to 70 and farmed the Hellfire Ring (which has no level requiriment and gives +45% xp bonus). Just equip that on a new char, along with everything you can buy/craft, and it becomes a god until at least level 25. After that things get progressively harder, but you shouldn't have any big difficulties. At about level 45 you should get back to your main and find a good 70 weapon with a socket, and roll a "minus level requirement". Put a godly gem on it, and it becomes an easy faceroll again up to level 60. After that I was just doing bounties coop (it's a lot easier to find public games after that level), and would hit 70 in a few hours.
Just do bounties, because rifts are too hard for leveling and campaign gives less XP. Also, you will hit 70 with tons of rift keys for later use. And don't forget to spend your paragon points as soon as you create your new toon.