I was surprised that no one suggested the possibility that you're over training until snowdog did so late in the thread.

Last month I went through a period of lethargy and it made no sense at all to me. My spring and early summer buildup seemed perfectly prudent but then-- all of the sudden-- my performance started to suffer. On training rides I couldn't get my heart rate up to where I wanted it to be no matter how much effort I gave. On one of my club's weekly ten mile time trials I think my average HR was something like 160! Plus-- my time was over a minute slower than it should have been on a night of very good atmospheric conditions. I was already depressed-- but the poor TT was the final straw. I took a few days off and when I returned to the saddle (a new saddle which as it turned out gave me more problems) I needed to take more time off still.

This week I'm back at it (with chamois cream) and my heart rate is back up to where it should be on my rides and I'm taking on the hills of the finger lakes where I live with great enthusiasm and vigor.

The advise you've been given on diet in this thread is good. At 280-- you're probably not getting quite enough carbs, IMHO. But your body usually tells you when you are under-nourished in the form of hunger-- and you didn't say in your post that you were hungry. Are you? If not, then it could be a case of too much too soon.