According to Official Guinness Records,
The fastest marathon in a superhero costume (male) is 2 hr 27 min 43 sec and was achieved by Matt Gunby (UK) at the Virgin Money London Marathon, in London, UK, on 24 April 2016.Inspired by seeing this record broken at the 2015 London Marathon Matt completed the course dressed as Wonder Woman. This was also Matt's first ever marathon.
For a complete list of 2016 records, please visit 2016 Guinness Records in United Kingdom.