The four-day Lindum Spirit endurance festival begins on Thursday at Market Rasen racecourse.
The horses have to undergo fitness checks by a team of vets before the events, including relays and a 100-mile race held over three days.
Organiser Thay Stephenson said: "It's the equivalent of marathon running for people - it's an extreme sport."
More than 300 riders will compete in the event and organisers say competitors tell them it is one of the best venues around.
Mrs Stephenson said: "Lincolnshire has a huge amount to offer in the way of riding.
"There are huge areas of open countryside, and an extremely good system of bridleways - we're lucky to get such co-operation from the land owners."
The event finishes on Sunday.
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