What does the postcard reveal about McCandless' journey?

From the text, we can see that Chris has managed to make it through the first leg of his journey. He's in Alaska, he's planning to return, and we can read that he is happy between the lines. I personally think he is being sarcastic when he say's, "If this adventure proves fatal and you don’t ever hear from me again", but I guess we will never know.

Chris does, however, concede that he is walking into the wild, a place that anything can happen.

April 27th, 1992
Greetings from Fairbanks! This is the last you shall hear from me Wayne. Arrived here 2 days ago. It was very difficult to catch rides in the Yukon Territory. But I finally got here. Please return all mail I receive to the sender. It might be a very long time before I return South. If this adventure proves fatal and you don’t ever hear from me again I want you to know you‘re a great man. I now walk into the wild. Alex. P
POSTCARD RECEIVED BY WAYNE WESTERBERG
IN CARTHAGE, SOUTH DAKOTA