67-Year-Old Steve Martin -- SHAPING UP for My Newborn Baby

Hoping to stay alive long enough to see his newborn kid graduate elementary school, 67-year-old Steve Martin worked on his fitness by hitting the road on his workout bike this week.

Martin's rep just confirmed that the actor's 41-year-old wife Anne Stringfield gave birth to their first child back in December ... but the couple hasn't revealed the sex or name of the child.