I got 80 farming pretty fast doing a few farm runs every day. Do toadflax or snapdragon herbs at all 5 patches, then do the best trees you can afford. Maples & then your best fruit tree (papaya is really good) is a great way to gain daily XP, but you can even plant willows if you don't have a lot of cash.
Also make sure to plant 1 Calquat tree per day when you get to level 72 farming. You can plant it once per 24-36 hours, and it gives you 12K xp every single time you harvest it. You can only plant at the Tai Bo Wani village on Karamja, and it is very cheap to get the guy to watch over it to ensure it never dies.