Diana Nyad -- PULLING A FORREST GUMP ... I'm Gonna Walk Across USA

She already swam from Cuba to Florida ... and now super athlete Diana Nyad tells TMZ Sports she has a brand new goal -- she wants to pull a Forrest Gump and trek across the USA by foot!!

Nyad was out in L.A. on Monday where she's getting ready to celebrate the 1 year anniversary of her historic swim -- when she told us all about her big plans.

"I just decided ... two years from now, I'm going to walk across America ... and get 1 million people to do it with me!"

And what's her motivation? Diana says she wants to whip America's fat asses into shape!!!

Check out the clip!