i think they originally quoted ~50 hours of play time to do the entire storyline. Gathering you can likely get 50-60 in a day or less if you have flying unlocked for the unspoileds.
I think Dungeons are the best way and combine it with Main questing + side questing. Although side questing can get a bit tedious. The FATE's aren't worth it atm.
FATEs seem to give less XP at level 50 and above. In the first zone I was only getting 8k-12k per FATE.
The XP amount is really low, like 14-16k. Some dungeons give me about 200k XP with rested XP + FC XP boost.
the dzu fate is worth 45k, there is a turtle type dragon fate worth 50k, and a few string fates that are 8-9k-17k, and probably more then that but I haven't done much, so if you can get into a fate train and kill them fast and have flight you can kick the crap out of dungeons with killing these fates within 5 minutes each if you are a DPS since you have 10 to 30 minute wait times for dungeons but if you are a tank or healer it would be faster for just instaque into dungeons
If you want to hit 50-60 the fastest do the following: Do your story quests until you hit a blockade of having to be higher level. Do every single quest that has the aethercurrent as a reward first. Find each aethercurrent (use the compass) before doing any other quests then the ones mentioned above. (some quests will require others before opening up, but most will be able to be picked up once the zone is fully unlocked. aka you did the dungeon quest) After flying is unlocked do quests until main story can be picked up again. Once your main story quest is at the point you need to be 60 and you're 59. Do Library non stop till 60. It's roughly 500k per run with rested. Pretty much rinse and repeat until your first 60. Currently the fastest way to level an alt is getting it to 51 and just chain dungeons. These give an excessive amount of exp per hour compared to the rest at this point. If you don't want to do it by hand (aka us botters) your best bet is to chain fate bot in Northern Thanalan doing those pre 50 fates since they still give a lot of xp and the mobs have significantly less hp.