Getting hold of Roy Claytons book on the Challenge of the Welsh 3000s, is well worth the effort.
He gives the potential walker or runner, loads of helpful advice on finding the best route around the three outlying areas.
Probably the biggest reason for failure, is not drinking enough fluids, and becoming dehydrated, as its a walk approaching 36miles in length, when you consider you have to walk up either Snowdon or Foel Fras.
Taking on enough fluid is vital, and eating small high energy snacks, on a regular basis.
Route finding is obviously an issue, but if you do not drink enough fluid during the challenge, then your more likely to get muscle cramps, bringing on extreme tiredness sooner than later.
Knowing what to take, and leave at home, to lighten the load on your back, is also important.
Just get a copy of Claytons book, its a great read.