The real training kicks in

My long run yesterday was 8 miles and it was hard. This surprised me and made me wonder if I'm up for this race in 5 weeks. But then I thought back to when I was training for the half and I think it was about this same mark that the long runs started getting difficult. I had the same thoughts then but despite my worries I finished the half and even managed to do better than I had hoped. So I'm trying to ignore that little voice of doubt in my head and press on. Today I am a bit sore from yesterday's run. It's a rest day for me and tomorrow I'll be running again. Why is it, then, that I feel like I could really use another day off?