I remember the discomfort most clearly at petrol stations.
Ale and I were riding motorbikes from Los Angeles to Vancouver. I was training for the Berlin Marathon, so each morning I'd go for a run before we got on the bikes. By the afternoon, after hours riding, my knee would be so stiff that getting off took some care. I'd stand beside the motorbike, slowly straighten my leg, walk around until it eased and carry on.
The following morning, I'd go running again.
I'd finished the New York Marathon with my buddy Steve in 4:39, then taken twenty-one minutes off that time in London. Berlin was next. I wanted a marathon time that began with a three. Even 3:59 would have been enough. I'd never been a natural distance runner, but I was improving, and I wanted to see how far I could take it.
Distance running had taught me to stop negotiating with myself every time something became uncomfortable. I'd learnt to trust that discipline, and it had served me well.
The training was going well, which probably explains why I kept dismissing the pain. I was running faster, feeling fitter and convincing myself that my knee was simply stiff from all the riding.
Eventually, it gave way during a run. I stopped, gathered myself and tried again. It happened a second time.
When I got home, I went to see Sam Church. He identified a torn meniscus and operated, which meant missing Berlin that year.
After the operation, I asked him whether I could try again the following year.
He said I could.
Then he explained that if I continued running and training as I had been, I might eventually need a knee replacement.
I'd spent months thinking about how many minutes I could take off my marathon time. Sam was asking me to consider what that ambition might cost me in the years that followed.
I've thought about that conversation many times in business.
I've worked with some extraordinarily determined people. Their ability to push through difficult circumstances is often what makes them successful.
Yet I've also seen how success can make us less inclined to question the direction we're taking. When a plan is delivering, confidence grows and the instinct is to keep pushing. It's difficult to challenge something that's working, particularly when so much has already been invested.
The trouble is that we tend to judge those decisions by the results immediately in front of us. The consequences of continuing in the same direction may not become apparent for years.
It's made me more conscious of the questions we ask when things are going well. Are we so focused on achieving the next target that we're overlooking what pursuing it might mean further down the line?
I never ran Berlin.
I still wonder whether I could have broken four hours.
What stays with me, though, is how close I came to making that the only question that mattered.