Naturally, with the Koch method, you'll be copying random groups of characters, rather than words, until you've mastered the entire character set. These groups of are of random length, rather than a constant stream of five-character groups, to ease the transition from random groups to actual words. Yes, there is a difference in the rhythm and "feel" of words and random groups. Once you've become accustomed to copying words, you should start copying sample QSOs, which are the format of the
amateur tests. Pay special attention to callsigns, locations, and numerals; these are the types of things that can form questions on the test.