Hi Marja, it sounds like you are doing great, good effort.
I think it's attractive for some people to use a heart rate monitor and don't disagree with this advice. On the other hand, just to say that plenty of people operate without one, by using the ' perceived rate of exertion' idea.
The kind of run that you are talking about where it is supposed to be just possible to hold a conversation is a lactate threshold run, and is by definition a little uncomfortable. It's not maximum effort, though.
Also, you need to be doing plenty of walk/run combos well below this pace, where you are running much more slowly/easily. In time, you will make adapations and all your runs will start to feel easier/ you will be able to run more and walk a bit less.