Adrian Corboy set off on the epic trip with 43 others on Thursday morning.

He was a late call-up for the Mongol Derby, replacing top thoroughbred trainer Ciaron Maher who broke a leg when he was thrown from his Caulfield Cup-winning horse Jameka last month.

Mr Corboy, 37, is Maher’s horse breaker.

Brother Marty Corboy said Adrian, who usually spends five hours a day in the saddle, had lost eight kilos in preparing for the steppes of Mongolia.

The derby involves competitors averaging more than 100 kilometres per day as they ride Mongolian ponies, whose heart rates are monitored for potential penalties if they are too fast.

Joining Adrian Corboy in a team is Maher’s sales and communications manager Annabel Neasham with funds raised from their deeds going towards Melbourne’s Peter Mac cancer centre.

Also participating is Kathy Gabriel from Benambra, who manages a station in the high country.

The race is expected to finish at the end of next week.

You can follow the progress of the competitors at www.mongolderby.com