Possible Clues:

• Horse (or person) that can go the distance
• Tenacious horse: famous name carrying you
• Tenacious type: guy about to make comeback
• Horse with endurance (informal)
• Strong horse as yet unreliable, right?
• One with endurance
• Distance racehorse
• Horse perhaps that's not much of a goer
• One unlikely to give up lead after overtaking you once
• Yes -- runs on way for runner who won't give up