It shouldn't take you long at all to get to 45.

Just keep using the Duty Finder / Roulette, get started on Guildhests as they provide great rewards as you go up in level, and battle through the main scenario quests for some great EXP rewards. Sometimes you can find a group of people doing FATEs, which is also a great way to farm Grand Company seals (useful for many things such as good gear, crafting materials, furniture, and more).