Keep in mind that Carlos Castaneda invented Don Juan out of whole cloth.
“Don Juan” must have appeared to Carlos among the stacks of the UCLA library since their records show that’s where he was when his books claim he was in the desert eating peyote with the Yaquis. He got his knowledge from reading the accounts of others.
You do know that you don't need to go to Mexico to get peyote. You could simply visit Warren Jeffs FLDS site ~ it's smack dab in the middle of the West Texas peyote range.
But you do make a good point ~ he'd had to have been sitting down to write and the preferred position is the squat (as I have been informed by users on the internet).