9 Mile Run Disappointment

I'm having a hard time making sense of my thoughts, but I'm going to post them here anyway...

After my last long run (8 miles) in November, I concluded that running a half marathon was not going to happen. My knee just couldn't handle it. My longest runs after that were 5 miles, and I even took a week off with no running at all. Yesterday, however, I decided that I wanted to try 9 miles. I got around to trying and completing 9 miles this evening. My knee seems to be okay so far, so I'm beginning to consider signing up for a half in January. There are exactly enough weeks between now and then for me to increase my mileage to 13.1. Maybe it's a sign. However, the voice in my head keeps asking if it will really be worth the registration fee and the small physical pains. I enjoyed my 9 mile run today and was proud of myself for having accomplished what I thought was impossible (because of my knee, because I couldn't run 2 miles a year ago) but those feelings didn't last long. I keep telling myself not to grasp after the feelings during or directly after the run, but it's hard not to. I suppose I expect each long run to "change" me in some way, to prove to me that I'm more capable than I believe myself to be. If the runs don't do that, then I begin to question why I'm running such long distances and end up in a funk. I feel as though I expect too much of a half and would ultimately be disappointed.