PARIS — In the days after finishing fifth in the men’s 20km race walk, Evan Dunfee struggled with how to think about his performance. He’d been fourth in the 50km at the 2016 Games in Rio, strode to bronze in that same event five years later in Tokyo. Now, after three years of reinventing himself as a 20km race walker, he hadn’t been able to go with the leaders in the final kilometres.