According to Official Guinness Records,
The greatest distance cycled in 48 hours on a mountain bike is 1,004.41 km (624.11 miles) and was achieved by Adrian Ellul (Australia) in McLennon Park, Walkerston, Queensland, Australia, from 9 to 11 October 2015. Adrian used a Giant XTC mountain bike for this record-breaking attempt.
For a complete list of 2015 records, please visit 2015 Guinness Records in Australia.