I've actually found Campaign to be a great source of xp while LFP or even if you are on for a short time. If you select the right targets, you can get anywhere from 500 - 2000 xp per battle (depending on length). It's all about the targets you choose to fight and your gear/skill.
Late late 50s and early 60s you should be aiming for mob strongholds like the ones they build during battle n' stuff (NOT fortifications). You could also fight imps, just may want to wait until someone of higher level pulls them. You can hit imps probably as early as 55, but unless you have some crazy eva gear, they tough to solo.
Mid 60s you can start going after Orcs and Yags, most don't have a whole lot of DEF/EVA, so can do decent damage to them (for a THF). Can also take Quads, just try to avoid Quad THFs and PLDs, either they have too much EVA or DEF.
It's also very helpful to Campaign with a friend, preferably of higher level. Right now I'm level 67 and I average about 2k+ per 20~30 minute battle when I campaign with my PLD friend (great for TA+WSing). Average about 1300 solo.
Edited, Oct 1st 2009 7:36pm by MinscandBoo
75+ DRG/BLU/DRK/MNK/SMN ~ 75 THF/SAM
Bastok: 10 ~~ Assault/Nyzul: Captain/100
ZM: Done ~~ ACP: Done
CoP: Done ~~ AMK: Done
ToAU: Done ~~ ASA: Done
WotG: 27 ~~ Campaign: Medal of Altana $$$$