According to Official Guinness Records,
The fastest marathon dressed as a fairy (female) is 3 hr 12 min 57 sec and was achieved by Harriet Chettleburgh (UK) at the Brighton Marathon, in Brighton, UK, on 17 April 2016. The attempt took place to raise awareness and funds for The Cystic Fibrosis Trust.
For a complete list of 2016 records, please visit 2016 Guinness Records in United Kingdom.