I agree with Waldo (as another LDWA man), it isn't too hard if you want to do it enough, but you haven't left yourself a great deal of time to prepare - and we don't know your age, job, commitments etc. etc.
Basically, get as much time on your feet as possible (every little bit helps, even an hour after work) and do long walks every weekend. Carry an overweight rucksack, train on the route and if you can do 25 miles you will manage 40 on the day. Be prepared for up to 20 hours if you aren't experienced and self supported, this also gives you time for a quick pint or two if you pass a pub on the day!
I agree with Kate about the Elan valleys, but if you drive to Beulah and then take the little Abergwesyn road there are a number of nice approaches to Drygarn Fawr ridge from the south. It is easy to make your own route once you have the OS map and from there you are highly unlikely to see anyone all day! It avoids all the visitors from the main Elan valley centre and a quick drive from Builth.
There is also a little forest café at Nant yr Hwch, grid ref: SN 883543.
If you make it to the top of Drygarn give me a wave!!
Thanks for that, about what I hope for and expect to make good time round N. Wales at the start and know what to expect once I hit Ceredigion going south.
Agree with your advice for Matt, Pembs coast I love (especially from Cardigan to St Davids), but also found the Milford Haven section interesting in it's own right. Going round Castlemartin ranges was a bit of a pain, but enjoyed the live firing demo (from a distance)!!!