The great thing about web pages is that although we all think they should be around for a long time, there's nothing stopping you experimenting - layout, themes, colours, content - absolutely everything.
There's a couple of questions to ask yourself, what do you like and don't like about your site? What do you want it to look like and behave? Once you've got those more or less settled, then you can start adapting a template or creating your own.
Once you've got your shiny new template, if you want to create new pages before changing your existing pages, then you can do that. If you want to update some of your existing pages, then you can do that. Just do what you feel you want to. There's no set method, no timetable you have to keep to.
A lot depends on what sort of menu you use. In my original pages, the menus were hard coded on every page. There's so many of them, I don't update those unless the page gets updated. The other 3 designs I use, depend on two different menu systems. It's a pain, but new and updated pages get added to both of them.
Sometimes I seem to have so much to do, not just the websites, that I spend more time thinking about what I have to do than simply choosing just one thing and at least making a start on it. It might be that's where you are at.