I started out with high impact, actually - way back in the 80s, when that's really all there was. Then I went into running, had to back off due to knee problems, and that's how I started the lower impact stuff. I really like the high impact best but have to be a little more careful now.
The thing that is good about Cathe's workouts though, is that they're not straight, full-on high impact. With her step routines, she mixes in step along with high impact jumping, for example, so it's easier to tolerate.
You can just ease into the high impact stuff though. Do what you can of it in her workouts, and then alter it to low impact moves that are similar when you've had enough.
And if you don't want to do high impact, then don't! That's okay too. Someone here on the forums can always help out if you have questions about how to convert a specific high impact into low impact. Just remember to enjoy the workout!