Herschel Walker -- I STILL DO 1,500 PUSH-UPS ... Every, Single Day

He's 52-years-old ... but NFL legend Herschel Walker tells TMZ Sports he still has an UNBELIEVABLE workout routine -- knocking out 1,500 push-ups every single day.

Walker was out in NYC on Tuesday looking jacked -- as usual -- when we asked how he manages to maintain his incredible physique.

For Herschel, he says the trick is easy ... just a simple 1,500 push ups plus THOUSANDS of sit-ups.