According to Official Guinness Records,
The fastest marathon dressed as a doctor (male) was 2 hr 53 min 11 sec by Dr. Ewan Jack (UK) at the Edinburgh Marathon in Edinburgh, UK, on 22 May 2011. Jack ran to raise funds for the Motor Neurone Disease Scotland charity and has raised more than £17,000 in all efforts.
Jack bettered the previous record by 1 hr 22 min 58 sec.
For a complete list of 2011 records, please visit 2011 Guinness Records in United Kingdom.