Like downtube said, there's just so many variables at play and too little info to make a good guess as to why you conked out at 24 miles. The thing is that some days you conk out after a few miles, and other days you feel like you can ride forever. A while back after a busy day at work being on my feet & walking around, up & down stairs, and lifting heavy objects all day, I tried to go out and ride 10 miles but I just had zero energy. I made it 5 before I decided to quit. However, the next morning I was able to do my usual morning 10.