You'll get this sort of thing from time to time if you're serious. My parents never quite understood why I put so much energy and time into something that to them seemed to have so little pay off. They were great parents but this was just one of those things.
I'd probably have told your uncle that it's my time and I get to waste it anyway I want as long as it doesn't hurt anyone and that I have no intention of quitting so if time wasting bothers him so much he might want to stop talking to me about running because he's wasting both of our time.
You aren't going to change his mind unless you do something like make a national team or get a shoe contract. Don't try. He's entitled to his opinion but he's not entitled to bother you with it. If he keeps on just tell him you aren't going to discuss this because there's no point to it.