as you may know, a letter is a manuscript that establishes interpersonal communication. The writer of a letter is telling about something personal about him, an experience, a memory, a feeling among others. In this case, it is correct that in a letter, the writer uses the pronouns "I", "me", "my". In addition, if the writer is on his way to Sherman, it is likely that he is participating in sherman's march and tell details of that experience in his letter.
