Possible Clues:

• Guest competitor who should do well in the long run
• Stumped by philosopher, I won't quit
• Distinguished performer carrying you once, an enduring ride
• Obedient dog, at times
• One with stamina needed when talking about flight
• Tough racehorse
• Following check, I'm not sure it'll keep running
• Celebrity impressing the old one with stamina?
• One enduring celebrity welcomes you
• One not easily giving up