You can stop to get more food/water. Your character won't do it automatically, but you can cancel an action any time you want, or you can click the 'start action immediately' button when you're half way through something else. In either case, progress on the action will be saved, and you can resume it where you left of later.
That being said, I do agree that the UI needs work, and clearly it may be benficial to add a tutorial which explains things like this.