Tempo runs are hard

I had an 11 mile run scheduled today, 1 mile easy and 10 miles @ marathon pace. I am using the training paces for a 3:40 marathon time, which makes the MP 8:23, even though I am planning to run at least 1/2 race no faster than 8:35 (which would put me at an 3:44:50 finish time, good enough for a BQ). I ace the speed drills and the long runs. In fact, I am ahead of the schedule with these, and always run them faster than I am supposed to, without feeling completely drained and exhausted. However, the tempo runs are much harder for me. Today in particular I struggled. My eyes were glued to the Garmin checking the mileage and the pace. Today I could not get into the groove. What's really interesting is that on Saturday I ran the last 10 miles of my 20 miler on hills @ 8:23 pace, and today I struggled to run 10 flat miles (OK, slight incline for the last 5) at 8:20. How could this be? I had 2 full days of recovery, my legs felt good, I took a gel and electrolytes during the run... I can only think of a couple of reasons: 1) I ran on snow, which, although nice for my feet and knees, potentially made running more difficult, 2) It was 12 degrees and windy (my water bottle froze), and 3) the adrenaline of running on the Boston Marathon course may have made me speedier on Saturday. Regardless, it's a bit disappointing to struggle with running 10 miles @8:20, when, even before starting my training for Boston I was running 10 miles @8:00 with no problem...I have another 11 mile tempo run scheduled in 2 weeks, same pace and I am curious how it is going to feel. Nonetheless, I think a tough run was in order as I believe during these tough runs one builds the physical and mental strength which are going to be much needed after mile 20.