Sore Quads.

Yup, Sasha is right. Soreness is due to muscle micro-tears, not lactic acid (for the most part). And I had the exact same thought as him in terms of your long run being too long based on your other training. Like he said, long run shouldn't be more than about 30% of your weekly mileage (40% is absolute max). If you don't want to increase weekly mileage but want to do a marathon, the run-walk Galloway preaches is the only way to go.

That being said, increasing weekly mileage will improve your speed a lot more than run-walk.

By the way, Sasha has superhuman quads- even most elite runners have very sore legs/quads after St. George.