According to Official Guinness Records,
The fastest half marathon run backwards is 1 hr 40 min 29 sec by Achim Aretz (Germany) at the 19th TUSEM August Blumensaat in Essen, North Rhine-Westphalia, Germany, on 28 November 2009.
Of the 464 total participants in the marathon (all of whom ran forwards except Aretz), Aretz finished in 123rd place overall. He was witnessed and videotaped by Christoph Diehl and Jan Verhoeven, who ran and cycled the half marathon behind him, respectively.
For a complete list of 2009 records, please visit 2009 Guinness Records in Germany.