The "Hot" rapper was clowning around with his boys Thursday evening outside the Sunset Marquis in West Hollywood when things turned into a fitness challenge ... as they often do.

Thug bet one of his pals he couldn't run up and down the street -- which is on a fairly steep hill -- and pulled out his money to show him what was at stake.

The guy accepted the challenge, and despite getting roasted for his running form -- or lack thereof -- Young Thug ponied up the dough. However, he made sure to rub it in by running the hill himself ... without all the huffing and puffing.

What makes Thug's running even funnier is the fact he's rocking super-skinny jeans

BTW, he's got a show Friday night at Avenue LA. No word if he's jogging to it.