Just go for transparency, not overexplanation or false self-deprecation if you are the shit.
Someone asks you your time, reply with the time. If they are overimpressed, just let it be. Don't encourage them, but don't say it sucks compared to what you used to run or what pros are running because a hobby jogger is lurking in the midst and it is like a thin person complaining they are fat.
If they are nonplussed, don't explain how good it is (I think this goes without saying.)
Possible answers to "Is that good?":
It's good for me, not a Kenyan.
It's good for an average person, but average for a really good person. (both tooting your horn and being self-deprecating at the same time)
I'm going to the Olympic trials, bitches--but not gonna win.
Yes.