The French rider, who is based in Crudwell, Wiltshire, hopes to complete the 26-mile run in four hours.
He will be running for the Get Kids Going! charity that provides lightweight wheelchairs to enable disabled children to take part in sport.
“I love sport and I think Get Kids Going! is a great charity,” the three-star rider told H&H.
“Preparing for the marathon has been a great way to get myself fit for riding my horses this season.”
The London Marathon takes place on Sunday, 26 April. To sponsor Leonard, contact him through www.goerensevent.com.