Heavy D -- Fitness Crusade Before Death

Heavy D hadn't given up on his health before his death -- TMZ has learned, the rapper was serious about battling his weight up to the very end ... running nearly everyday, and frequenting several L.A. gyms.

One of his most recent trainers -- a guy named Tony, who helped Heavy D lose 150 pounds in 2008 -- tells TMZ, D stopped in to Ultra Body Fitness in West Hollywood 3 months before his death ... and began to re-strategize his training program.

Tony tells us, the rapper looked like he hadn't worked out in a while and wanted to get active again.

At that point, sources close to Heavy D tell us, the rapper embarked on a daily running regimen and frequented several other nearby gyms to diversify his exercise experience -- incorporating everything from hiking to boxing.

At his peak, Tony tells us, Heavy worked out 6-7 days a week.

When he died, we're told everyone at Ultra Body Fitness was shocked -- and the members even erected a small memorial in Heavy's honor (below).