You're right--I've never been fitted for a bike and have no idea what I'm doing. As good a deal as the Kona Jake is (and from extensive googling it does seem like it is fairly common for people around 6' to ride the 56cm one) I think I'm going to just get this Bridgestone MB4 Mountain Bike
because it's really cheap and has new road tires. It looks a bit smaller than the jake even but the flat handlebars will make for a more relaxed riding position I think. It just seems like it's in good condition, and although it appears to never have been even a remotely nice bike, it'll do me fine. Thanks for all the help everyone!

Also contemplating buying the jake, trying it out, and then flipping it if it doesn't fit me...