The 1910 Tour de France
The Tour de France started in 1903; but it wasn't until 1910, seven years later, until race organizers included the Pyrenees in a stage.
Stage 10 of the 1910 TdF was a beast: 326km (203 miles) that started at 3:30am and finished (in a sprint) 14 hours later.
I can't imagine what riding the Tour de France course would be like today, much less in 1910. Back then, the bikes were heavier and didn't have proper gearing, the roads were terrible, riders wore wool clothing (no Dri-fit), and they weren't assisted by cars or other support vehicles.
The Tour de France is the pinnacle of competitive sports. No other race or feat is more demanding. Watching this video gave me a true appreciation for the old cyclists long forgotten.