This is the highest you can get your level to legitimately. Getting there will require you to accumulate 5985083 total experience. Though this may sound like a lot, there's more than enough experience available in this DLC to make it there. Any experience earned from the original game or other DLCs count towards you experience total. Therefore if you haven't completed missions from those stories feel free to do so once download this DLC. With completing missions and killing enemies, you should level up very quickly. If you want to really expedite the process though, you could try to join a group killing Crawmerax as that boss and his minions give boat loads of XP.