13km is not too far to walk, especially if you're pissed off and on a mission and you know where you're headed. You've got all night - 13km is nothing. And if you've taken the pills (and I say this because a good family friend did this last year in north Qld, took the pills and crawled under a bridge to die), then it's only a matter of time. Sleep time.
I realise I am playing the worst devil's advocate ever here. I realise this flies in the face of most sentiments previously expressed, including my own. BBut I also feel that if this is a true crime-busting forum, we must discount all possibilities.
I like a good alternate theory, but this still remains highly implausible, imo.
For example, surely you concede the scratches on GBC's face were not from a razor?