Given where you are with your age, having a pretty good idea of everything, I'd say your best place to start, would be to roughly block out the places you are supposed to be able to traverse, using simple cubes and other primatives, before doing/delegating any actual modelling. This will allow you to visit and get a sense of the scale of things and spot imperfect layouts, before spending a lot of work, only to find that you have to back up.
You could give it a go, beginning with the minimum requirement .blend file attached to this post. (You can export an age out of it, right away - it has a link-in spot and a pair of plain platforms)
You'll need to acquire a "sequence prefix", i.e. unique ID for your age and enter it into the "book" text section in the .blend.
This can be done here:
http://ulm.wolfshire.co.uk/