For level 70 you are required to get a whopping 24396499 XP (no that's not a typo!)
Just playthrough the game as normal completing every quest and kill everything you see. I was level 61 when i had done everything apart from a couple of misc achievements.
To grind the remaining XP you can abuse the continuous quest given by Nina at the Makeshift Hospital. She always wants bleach (Must be planning on drinking it or something) and the good thing about this version it hasnt been patched like the PAL version. You can head over to Aubrey at the workbench below the Motel Deadzone and buy as much bleach as possible and head on back to turn in.
Simple as that.