Having doubts

My runs have not been great lately. I can chalk up the difficulty with the long runs to the increase in mileage but even my "short" runs (typically 5-6 miles) aren't going as well as I'd like them to. My legs just feel tight much of the time. Yesterday my knees were actually sore, which is never a good thing. This makes me worry. Now, they were fine as of this morning so that gives me a little confidence that I'll shake this all off but there is still a little nagging doubt in the back of my mind. My long run on Saturday will be 10 miles. After that I start to taper. I'm sure I'll make it to the race but these recent problems have me wondering if a full marathon is really within my reach.