Here's my two cents on top of Bob's:

My recollection of the sequence at this point in Clemens's life is that he
had decided to take a job offer at the ENTERPRISE *before* he left on his
long walk, so I don't think it's a case of Sam wandering in the desert and
coming to an important decision because he had a "breakdown."  He knew
before he headed out that he would now try to earn a living as part of a
newspaper--a situation with which he was familar from the days working on
Orion's papers.

Also, I think it is important to remember that for much of the 19th
century the average American male took up many occupations in his
lifetime--because he was a failure at the first several efforts (John
Clemens, George Washington Harris, and U.S. Grant are other instances
that come readily to my mind).  That was what part of the meaning of the
"lure" of the American frontier: to try again, maybe even leaving one
place for another one step ahead of the sheriff trying to collect for
debts.  Sam Clemens had been on his own for a long time and had enjoyed
more success than many when he decided that mining was too damned dirty
and difficult compared to scribbling the news.

None of this means that the question is not interesting in psychological
terms: what kinds of hopes and fears did Sam have during the long walk--a
good time to meditate.  But "breakdown" strikes me as melodramatic and
incorrect for a way to describe the moment.

Jim