I saw this on HBO in the mid '80s and I loved it. Tommy Lee Jones was so cool and I had no problem buying the premise. It was kind of like Assault on Precinct 13 or The Warriors in its cartoonish depiction of violence. I remember especially the part where he's in full military garb and grease paint on his face with an arsenal of weapons around his body and he tells a woman to leave the park because its filled with "thugs and perverts and weirdos." Check it out if you can, it won't change your life, but it's good.
Review of The Park Is Mine
