Triathlons 

Running is the last leg of any common triathlon.  Triathlons usually consist of three events in this order: swim, bike, run.  Distances are event specific. 

For an Ironman triathlon, the running portion is a full marathon (26.2 miles).  For other triathlons, the running portion can vary from less than 2 miles, to 13.1 miles.   

Triathlete is a great place to investigate competing in a triathlon.