You see, on starter accounts that have yet to pay for a subscription, any XP you gain past level 20 is "banked," up to a maximum of 10 levels. For all intents and purposes, you're stuck at level 20, and don't gain any more XP past the 10 banked levels, but subscribing to the game automatically grants you those levels. Additionally, any XP you do obtain is calculated as if you were level 20, and no higher.
Some of them can only be battled by characters of a specific faction, but can still be turned in remotely by a character of the opposite faction. It took about a couple weeks of constant turn-ins to get to level 39 with this method, but I managed it, and I'd be lying if I didn't admit I was proud I did!
(Oh, also if anyone knows another way to gain XP on alts where mining and herbalism aren't possible, feel free to share it with me so I can expedite the process a bit more, that'd be great, thx)