Perseverence and Doubt

Today I ran 18 miles, my longest and farthest run ever. The first 13 miles were fine, but the remaining 5 were a real struggle. I was still able to maintain my pace and finish in 3 hours and 12 minutes, but it wasn't easy. Multiple times I thought that there was no way I was going to be able to finish. It is not that I thought about quitting, I literally thought that my body would do it for me.

Perseverance is the only thing that got me through, but even though I finished, the struggle did mess with my confidence and put a seed of doubt in my mind: How in the world will I ever be able to finish my 20 miles run in couple weeks? And if I don't finish that one, what are the odds that I will do well during the actual race? I just hope I feel stronger during the next couple weeks. We will then go from there...