It depens on how much you eat and what kind of person you are. There are people who are very active, but others maybe not. And then i think 30-40 minutes will be enough.
It’s an effort that you will thank yourself for. You can still work in other exercises if you are able and want more progress & improvement
Depends on so many things. What you are trying to achieve, what was your previous workout regime, how fast you are walking. I think better matrix is can you improve every week on whatever you were doing the week before? Can you complete a km in 30 secs less time? Can you walk for 10 mins longer?