Sometimes when you're trying to make a difficult decision, there are figurative signs pointing you in the right direction.
As noted before, I've ridden the MS 150 (a 2 day, 170 mile bike ride from Houston to Austin) five times and finished it four times. Back 1993 I first rode the MS 150 with some friends, including my incredibly handsome twin brother. It had been around 10-15 years since I had ridden a bike, but I was young and moderately athletic. I rode on some short rides around Austin to get ready, but my training was interrupted by a bout of the flu a couple of weeks before the ride. However, I recovered and felt ready to go in time for the ride. Or so I thought.
On the first day my group started out. The stronger riders in our group rode ahead, but several of us took a more leisurely pace, pausing frequently to take pictures. We stopped at all the break points to drink fluids and eat snacks (power bars, bananas, etc.).
Around the 50 mile mark, I started feeling bad. I had a fever, chills, and I felt very weak. I had to make frequent stops in between the break points and rest, so I told the group not to wait and that I would catch up.
Around the 80 mile mark, I made another stop on the side of the road to rest. I felt weak, and my whole body ached. I stood next to my bike trying to decide whether I should continue or not. I had no idea how many more miles I had left or if I could make it, but I didn't want to quit. I went back and forth, alternately between wanting to give up and talking myself into going on.
And then I felt a stinging sensation in my right foot and lower leg. I looked down and saw that I was standing in an ant pile, and my foot leg were being bitten by fire ants. I paused, took a deep breath, and thought, "Well, there's my sign." And I called it a day.
I was only 20 miles from finishing the first day, but I'm fairly certain I wouldn't have been able to ride the 70 miles on day 2. Plus, I knew how to read the signs.