The Minchas Chinuch says you are only yotzei the mitzva of eating on erev Yom Kippur with the kind of eating that is assur on Yom Kippur. Therefore you need to eat at least a Koseves within 2-8 minutes depending on
the shiur you hold for “K’Dei Achilas Pras.” We are noheg to have a seudah where we wash and eat bread in the morning. The Sdei Chemed says bread is required, although the Minchas Chinuch argues and says any food is okay. Some have the minhag to dress in Shabbos clothing and have Lechem Mishneh.
There is a Minhag Chasidim to eat the entire day. Some suck candies. It is said that the Maharil Diskin sucked raisins all day. This is in accordance with the opinion of the Ksav Sofer that any eating is a mitzva.
When talking about the mitzva of eating it always says “eating and drinking” and the inference is that drinking is also part of the mitzva. (Piskei Tshuvos).