I highly recommend learning a bit of technique before jumping into an advanced KB workout. Something like Katalin Zamir's instructional video, Kathy Smith's Kickboxing workout (which teaches technique), or Powerstrike Millenium 3 (which teaches technique and builds up combos more gradually than other Powerstrike workouts).
It's important to be able to punch without locking your elbows (go to about 95 percent extension) and kick without locking knees, and to do a good chamber for kicks (bringing in a bent leg before kicking out with the lower leg).
Also, when doing punches, your whole body is involved, as the power comes off the pivoting foot through the leg, in through the core and up into the arms. When one is first learning KB, this coordination can feel very unnatural, and trying to do it at a faster speed (as I assume the newer KB workout may be) can lead to some knee torque because the movements aren't coordinated.
Also, once you do get Cathe's KB workout, follow Lorraine and Cathe for form (Jai punches too low, and has a strange 'frozen shoulder' type movement for hooks and uppecuts, unless her technique has improved since the last KB workout she was in).