Several of my regular bicycling partners have started a racing team this year called Wild Card Cycling. This weekend we planned a long (even by my standards) training ride together. The plan was to do a century (100 miles). My previous longest ride of the year was around 50 miles or so, so this was a big jump. After 75-80 miles I bonked (*).
Bonking is basically the same thing that marathon runners refer to has "hitting the wall." It is the point at which your muscles run out of glycogen and can no longer continue to function at their current capacity. In a matter of minutes you become incredibly tired and your heart rate skyrockets. You can keep going after you bonk, but your performance drops significantly. Your body has no stored carbohydrates so it has to burn only fat, which is a poor source of energy.
The only way to avoid bonking on really long workouts is to continue to eat during the workout. I was eating during the ride yesterday, but apparently it was not enough. After 4.5-5 hours or so my body stopped cooperating.
Fortunately, another guy bonked at roughly the same time and we were able to ride together the last 20 miles into town. It seemed to take forever. I ended up at home after 96.33 miles and decided not to do another 4 miles. I didn't have it in me. I was past the point of enjoying myself anyway. There will be other days.
This morning I did another 35 miles with the group. It took 20 or so miles to work the lactic acid from yesterday's ride out of my legs, but I felt pretty good by the end. It was dark and rainy right until the end of the ride then the sun came out and it's now a beautiful day. If only we had waited an hour.