While I just purchased 6 new ones, I have only done Itreads #13,19,21. 19 is killer, 13 very doable and 21 very good. These are all intervals,different though. 19 has a couple of hills and 13 is flat,21 has a little incline but not much but you dont get walking breaks at all,in the other two you do. I love them all. cant wait to try my others. I have 12 in all now, so I have 9 more to try.