Leadville — Runners and donkeys have been busy training in Leadville before a big 21-mile run this weekend.

Racers from all over the world will be participating in Colorado’s oldest sport, pack burro racing.

“Burro racing is the only sport that’s indigenous to Colorado,” Brad Wann with the Western Pack Burro Association said.

Burro racing is where donkeys are saddled up and carry a pick, shovel, and gold pan like they did in the old mining days.

“There’s 4,500 feet of elevation gain and loss during that 21 miles over some of the worst footing known to man,” Dave TenEyck with Leadville Boom Days said.

But burro racing is just one part of Boom Days, which is in its 70th year and runs from Friday through Sunday.

Festivities include parades, costume displays and pancake breakfasts. But it’s the donkeys that really grab people’s attention.

“A burro can run a four-minute mile if you can hang on,” Wann said. “It shows you have the ability to navigate to negotiate.”