Even if you are a really good cyclist, I would assume you still want to be humble about it. I get asked this a lot, but don't really know how to reply in a non-douchey way.
Although it is highly unlikely that someone would make that comment to me (I typically ride 12-17 miles, at 12 mph,5 times per week), if I was told that I'd simply say a thank you followed by "I'm just a cyclist."
I know I am still far from becoming a good cyclist but if someone would tell that compliment to me, I would simply smile and say thank you. Deep inside that compliment will fuel me to do my best
I wouldn't know, it never happens lol. Even if I were overly good, I generally keep my hobbies to myself as nobody I know shares the same interests as me. Any athletic people that I do are generally much more advanced than I.