Hey Haylie,
Good news: not everything that is called "cardio" has to be just running / jogging or hours on the treadmill. You can utilize almost every exercise that includes a bigger area of your body to burn fat. Just a few examples: Kettlebell Swings, Lightweighted squats (if you can do them with your knees- might prefer front squats as the knees stay more backwards), spider crunches (as a bodyweight exercise), or also box steps (which aren't too taxing on the knees).
Cardio is all about getting the heartrate going, either for en extended period of time with some medoim pace cardio - which is not helping you with your goal to build muscle though, because this long and slow cardio rather will make you loose muscle along with fat.
Cardio is also everything that goes into the realm of conditioning.. meaning: High intensity interval training (HIIT).
And you can basically execute a lot exercises (of course with lower weight or bodyweight only) if you do this for 10-15 minutes in really intense intervals of burst phases and rest phases. (i.e. 10 second - give all you got, 20 second rest - do this for 10-15 minutes. Future goal: 20 seconds - give it your all , 10 seconds rest.. rinse and repeat)
You might even want to cut down a bit on the long walks. 45 minutes every other day is quite taxing on the knees and ankles. Which is not a problem if you wouldn't have issues with them. You could try to do 1 day of long walking, best not after a leg day, and for the rest try as described to find exercises that you can turn into HIIT cardio.
And as John wrote - nothing is written into stone - except for physical basics and how the body functions in general - and all of us have their little injuries or special requirements when it comes to our own bodies and have to adjust our training accordingly, and you'll have to find yours.
One thing though, as far as i remember Body-4-Life is mainly build as a gym / or personal trainer program. Since most of their exercises revolve around benchpress, squats, legpresses etc. I don't know how your setup and equipment at home is, but maybe you can find one that is more speciffic for bodyweight or at home training ? Unless you have all equipment needed of course :)