Depending on your own psyche, you may expect to experience several unexpected emotions, such as an uneasiness about being out there all alone, missing having other people around, and general insecurity about your own skills (even when you know you have no reason to be insecure.) Just be prepared to talk yourself through them.

Depending on where you're going, how heavily used the trail is, and whether the campsites you're using are designated by the land managers (thus making it less likely you're there alone), these emotions may be exaggerated or less severe. I know on my first solo, my mind was working hard to sabotage me - right up until I met my first hiker; then I was fine.