Most people sign up for some spin classes ... but "Biggest Loser" host Alison Sweeney is taking that 26.2 steps further for her New Year's resolution -- telling TMZ, she plans to run the L.A. Marathon this year.

She's got 76 days left to get ready -- the marathon is scheduled for March 18 -- but Alison tells us, she already got a head start ... beginning her training two months ago.

Sweeney says she plans to hit the pavement with several "Biggest Loser" alums on the big day. Now that's commitment.