I Want To Go To Cadair Idris But I'm Not A "walker" - What Should I Do?

Q. I want to go to Cadair Idris (walk around it and in it and stuff) as a major scene takes place there in one of my favourite books but I'm not much of a walker. I've walked across a glacier once and done lava-tubing with school once, both in Iceland, I've been to Brimham Rocks a few times and I climb the hill in Levisham in the North York Moors to the top in just everyday shoes fairly often (2-3 times a year I'd say - often for me). I walk around my city quite often but obviously I'm aware that something like Cadair Idris is a lot different. I have walking boots that I had to get for Iceland and waterproof trousers but that's it. What should I do/get in order to go there? My family (who are not walkers at all) will most likely be with me or my non-walking friend (I mean she can walk but she's not a mountain walker or anything).