''When we left thirtysomething, we all ran away from each other. Everybody wanted to prove they had a responsibility for its success,'' says The Byrds of Paradise's Timothy Busfield, the first thirtysomething regular to star in his own series since the show's cancellation in 1991. ''The competition came out in all of us. It was like trying to prove who was for real and who wasn't.'' With the lead in Byrds and steady work in feature films (Sneakers, Striking Distance, this July's baseball comedy Little Big League), the former Elliot ; Weston appears to be ahead in the race. Here's how his costars are faring: *Real-life husband and wife Ken Olin (Michael Steadman) and Patricia Wettig (Nancy Weston) are currently producing Kansas, an ABC drama pilot in which Wettig will star. Olin directed the acclaimed 1992 TV movie Doing Time on Maple Drive, and White Fang II: Myth of the White Wolf, his big-screen debut, is due April 15. Wettig returns as Billy Crystal's wife in May's City Slickers II: The Legend of Curly's Gold.
*Mel Harris (Hope Murdoch Steadman) has toiled in a string of TV movies (Desperate Journey, With Hostile Intent) while trying to launch a film career. She landed a role in the surreal black- and-white independent thriller Suture, opening next week.
*Peter Horton (Gary Shepherd) directed the pilots for Birdland and Class of '96. He returns to serious acting with this April's When Darkness Falls. ''(It's) a good bad TV movie in a genre I hate, disease-of-the-week,'' he says. ''I did it partially because I needed a job.''
*Melanie Mayron (Melissa Steadman) recently played an doctor in the Lifetime movie Other Women's Children and guest-starred as a cop on Lois & Clark. But as with Olin and Horton, what she really wants to do is direct; she has been behind the camera on episodes of Tribeca, Sirens, and Moon Over Miami. Would she ever do a thirtysomething reunion? ''If everyone else is willing, I guess.''
*Polly Draper (Ellyn Warren) appeared in the TV tearjerkers Danielle Steel's Heartbeat and Broken Promises. She just ended an Off Broadway run as an over- the-hill starlet in John Patrick Shanley's Four Dogs and a Bone. So would she do a reunion? ''What did Melanie say?''