His chances are excellent. Most people don't know that most professional athletes hold wrestlers in awe due to their commitment to year-round schedules and fitness and the fact that most must work through their injuries.
Brock has an extra advantage by being one of the fittest wrestlers who ever lived with relatively little damage to show for it. He proved his intelligence and physical control by putting on hundreds of entertaining matches without seriously harming anyone (guys his size are notorious for "working stiff").
Best of all, Lesner left the WWE in good graces knowing that he may have to fall back on wrestling. Pretty wise for a 26-year old.
The worst thing wrestlers have going against them is the injury thing. The year-around schedule make them have more injuries and injuries that would have you and me in a hospital for a week, they have to work through.