Tadpole Time Trial

Last night I rode the Tadpole Time Trial, a 10-mile bike along a rolling, wooded road. I felt good leading up to it. I had originally planned to drive to the start (about 6 miles from work), but when I got in the car I realized that I had 20 minutes to get there. It's relatively flat road, so instead of turning the ignition I got on the bike and road out there.
I started off great on the ride, I was riding smooth and easy, and fast (for me). I was well under my goal pace of 28 minutes. The last hill before the turn-around is long and steep in places, and that hurt some, but I felt like I would finish strong. The route back is more downhill than uphill.
I got down the big hill, and then just after the 6-mile mark there is a short, steep hill before a longish flat section. I attacked the hill in a big gear, standing up and cranking with everything I had. Then, about 2/3rds of the way up, BONK! The power plant just died.
I made it to the top, but I was hurting, and I couldn't accelerate on the flat at all. I limped along for the next couple of miles at an agonizingly slow pace, watching people who had started after me roll past. I put in a final sprint at the end, but even then I felt like I had no juice. I knew I could go faster than that, but there was just no power.
I finished dead last, in a disappointing 31 minutes. I've been thinking about why I tanked so badly: could it be the lack of gym time over the past month? Or maybe the nagging fatigue Sue and I have both been fighting for the past week? Nutrition? Hydration? I'm just not sure.
I do know that I can do that trial much faster than that, and I intend to do so.