I know what you mean about the apprehension in terms of walking around the block in your new boots. I feel much the same about my 12cm boots. At the moment I have no place I could do that, but if I did I would still feel nervous about it. As you say, you have to start somewhere and you’ve already managed a 100 metre walk in them -which is something I’ve yet to do in my 12cm boots.
it is funny how even just sitting in these lofty heels can help you get used to them. When I stand up to go make myself some coffee or make lunch, I’m automatically expecting the steepness and height of 12cm stilettos - it’s a mental thing but very useful for accepting the height and challenge of these heels