The 29-year-old won in a time of 5.58.53, becoming the first women to complete the Durban to Pietermaritzburg route in under six hours.
Steyn was the 17th person over the line at the Scottsville Racecourse.
The men's race was won by Edward Mothibi.
Bulwer's Bongmusa Mthembu came home second.
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