I know from experience that Snowdon is hard when unfit.
I did it last year with my son, and I struggled massively up the Rangers Path.
Hill walking is more about stamina and endurance, than muscle building, basically it's like walking up the stairs in a tower block.
Like any endurance sport, training should be progressively lengthened. Start with doing a distance that you are comfortable with, and then steadily increase the distance. Once you get comfortable doing an extended walk, find a route that takes you up and down hills, the journey down is as important as the walk up as this is a rest time for you legs, and then as before as you get more confident, increase the incline.
It's all about pushing yourself past your comfort zone, until you get to a point where you cannot reach it any longer.