Well I had a new experience happen yesterday and wonder what might have caused it?

I'll be sixty in two weeks but am in pretty good shape. I'm 5'5" and weigh 131 lbs but have high cholesterol. Can't take statins but do take 2000mg of niacin daily. (That's another story) I've been riding off and on for over twenty years and have been riding about three times a week since January. My training rides are typically twenty-five miles and occasionally thirty. Three weeks ago I did a 51 mile charity ride and finished it well and felt good when it was over.

Yesterday as part of Bike Ride Across GA (BRAG) I rode 59 miles. The course was moderately hilly, and the temp was 77 when I started and 86 when I finished. Humidity was 60%. I averaged 15MPH.

I ate and drank at all three of the SAG stops (PowerAde and water) and drank lots of water while riding. At no time did I feel dehydrated nor did I feel like needed food. However...I bonked big time in the last ten miles and the last couple of hills just about finished me. It was all I could do to put my bike rack on the car and get loaded and I was very dizzy for the hour car ride back home. Thinking I needed protein, I stopped at a Wendy's but that didn't seem to help at all. Once home, I collapsed for about the next five hours and remained somewhat dizzy for the first two hours. My quads hurt big time and I was just totally exhausted.

Granted the temps and humidy were quite a bit lower on the 51 mile ride a few weeks ago, but with the amount of riding I've been doing (although not real long rides) this has me a bit mystified and bummed out.

Any thoughts? Thanks.