Kellen Winslow Jr.'s NFL career might be over, but he's decided to channel his competitive drive and focus on DOMINATING a whole new sport ... cycling.

The former Pro Bowl tight end -- who played 10 seasons in the NFL -- has been training like a maniac ... already dropping from 245 to 215 lbs so he can compete with the best cyclists in the world.

"My goal is to go pro. With my determination I'm going to get there. I put all my effort into it. I absolutely love it."

Kellen says he's biking between 30 and 60 miles per day -- "It's what I have to do to keep up with and beat the guys I'm going against."

"Fitness wise, I'm up there with anyone. I just need more race experience."

The 31-year-old has already tasted success -- winning his first race ... and finishing in the top 10 in 5 of his last 6 races.