Nairobi, Feb. 28 -- Instead of throwing a lavish party to mark his 50th birthday, Leonard Ngunga, a cardiologist, chose a different kind of celebration. That was last month, and he did a 50-kilometre run.

"I woke up at 3 am to start that birthday run. It took me five hours to complete. You only turn 50 once. I wanted this milestone to mean something beyond myself," he says.

In the weeks leading up to the ultra-run (defined as any distance beyond 42 kilometres), Dr Ngunga trained intensely, averaging 100 kilometres every week. Every stride, every split, every kilometre was logged in an app on his iPhone, a digital diary of discipline and determination.

When we meet at his office at Aga Khan University Hospital, where he heads Cardiac Ca...