Golf Legend Gary Player -- 5,200 Sit-Ups Per Week ... At 78-Years-Old!!!

He's 78-years-old ... but golf legend Gary Player tells TMZ Sports he still knocks out 5,200 sit-ups every single week!!!!!!!!

This is no joke.

The South African stallion was making his way through NYC Monday when he told us about his incredible fitness regimen ... a regimen that landed him on the cover of ESPN's Body Issue in 2013.

And it's clear from the video ... even as he's pushing 80, Player has no intention of slowing down anytime soon.