In my opinion (not professional, just from experience)
1) I would say try to hit your maximum speed and endurance first to know where you are. Example, walk at the fastest pace you can or jog and see how long you can last without seeing stars. That will be your bench mark. Then when you start the program, you know which speed is safe for you to push yourself without getting hurt. Like me, I can only jog at 8km/h for 3 minutes (without fainting), so I know not to go faster than that when following the program. Or if I go faster, then I won't go longer than 3 minutes.
2) Warm up longer than 5 mins. That 5 mins warm up ain't good enough unless you stretch or do some kind of warm up before hand (which I usually don't). For me, I walk / hike for 15 minutes as warm up. If you don't warm up properly you risk getting hurt.
3) Go consistently. I know the program says 3 days a week but that is not enough to get you into habit. It's very easy to slack off. I suggest at least 5 days a week. The other 2 days just go for a stroll or a slow jog if you are up to it, but you must go and walk. Don't stop if you have muscle pain, but instead do some gentle stretching or go for a swim instead (slowly)
4) If you really cannot complete that particular day / workout. Don't be discouraged. Go back to the previous set and repeat the week. You will realise it'll be easier to attempt it again. Do it at your own pace.
5) Reward yourself. I find that motivating. Like if you get to week 5, you will reward yourself with a treat or a new pair of socks (that kind of thing)
6) Music! Create a playlist with all your favourite songs. I find that it helps when you are out of breathe and ready to give up. I love mouthing the lyrics and run at a faster pace during the chorus.
I hope this helps and may you have loads of fun doing it! But as to how you should start, I'll quote Nike and say Just Do It!

I am trying to get started walking and I want to walk. I just found out I have nodules on my lungs and shortness of breath. I need advise of where to start and how often I should walk

I found that 90 second jogging intervals was too much for me to start out. Once I found a plan that started at 60 second intervals, I was able to finally get started. The first week (day 3, in particular) is one of the hardest because your body will not be happy - try to make yourself just do it. It's always a challenge when you start a new week/increase the interval times, but that first one was the hardest for me. Listen to your body if you don't feel up to moving on to the next week - simply repeat weeks when you need to.