The easiest way to edit the content of a cell, be it a formula or a value, is to navigate to the cell and hit F2. Instead of moving around the cells in a sheet, the left and right cursor keys switch to stepping between the characters of the cell. Hit F2 again to toggle the cursor keys' behaviour, and they can be used to navigate across the sheet again to reference other cells in the formula being created.
And if in editing your cell your formula gets long, you can extend your formula bar beyond a single line. The thin divider immediately above the column titles can be dragged down the screen, and your full glory of the formula can be seen at once.