hi @themessygarden I can't do lunges or burpees either due to bad knees. I can do less deep squats which are apparently ok.
I have learned to substitute lunges with other exercises (I can do deadlifts for example, can do many of the glute exercises and I looked up some quad exercises that I can do instead of lunges) and have found that my legs are getting stronger. I did start off by just fastforwarding through the lunges and deep squats which was ok too although I didn't do any leg exercises!
if you search Caroline Girvan on you tube she does have some knee friendly leg days and you can then do those instead of the continual lunges. I find the HIIT difficult because I can't do burpees or lunges or curtsey lunges but I do alot of HIIT anyway. I think this weeks is all boxing, so it's doable. Again, I save the workouts that I can do and then substitute them.