In my case, exhaustion.
I figured I'd do 25 miles, tops. The plan was to leave early in the morning and ride further up the river than ever before. I left at 6:30, when the sun was already up... and so were the bugs. So, I had to ride with my mouth closed - you know, or else! But it was a pleasant morning. It was cool and there were a whole lot more people out there than I'd expect on a Sunday.
As the river coasted along the 91, I was reaching my furthest point to date: Weir Canyon. At Weir Canyon, the trail splits, one side proceeding down by the river, the other side climbing into a hill. I would have liked to stay with the river, except the side ended shortly thereafter. I had to climb. I was too stupid to shift, also I didn't know how far I'd be going... after a while, though, I shifted, because it went pretty damn far. But that was nothing, because after it climbed and climbed - it dropped. I zipped down into a ravine at nearly 30mph but then had to ascend again... dammit. Okay, so I didn't need this to tell me I'd never make it to the Tour de France. But after I climbed up again, the trail had beaten me enough... for a while. It resumed its graceful arc along the river, fairly flat, calm, with more bikers and joggers than I could believe. Yorba Linda folks.
I went for quite a while, looking for some landmark so I can say, "I went to..." So, when I reached Gypsum Canyon Road (which to you out-of-towners is past the 241)(which to you really out-of-towners is just one more street and, sadly, only a couple of miles further up the triail), I knew I had my landmark, turned around and headed back. I had put on some sunscreen for when the sun finally came out... but it wasn't cooperating. It was actually kind of chilly! I slipped in line behind a big group of riders who were doing an even 12-14 mph and followed them on back.
Here's a tip. Be careful about what sunscreen you use... be careful bugs don't like it. By the time I got home, 24 miles later, my arms and legs - all sunscreened up - were now covered in bugs!
And, for some reason, Vicky didn't want a hug.
But after I wiped off insect detrious, Vicky pulled her bike out and we went on a ride for the two of us. Now, it was just 7 miles long but keep in mind Vicky isn't as crazy as I am. She doesn't bike in her every spare minute. Anyway, after a couple of miles, my ass had had it! It was threatening to leave! And I don't want to know what life without an ass would be like - so I asked Vicky to turn back.
All in all, though, 31 miles. Not bad. Nearly 1/3 of a century. I wonder how stupid I'll be next weekend...