10:36 AM PT -- Johnson finished the marathon in 3:09:07 ... JUST missing his 3 hour goal.
Still, an AMAZING performance from a guy running his very first 26.2 mile race.
His average time per miles was 7 minutes and 13 seconds. Incredible!
Congrats!!
Jimmie Johnson doesn't need a car to go fast -- the NASCAR driver is running the Boston Marathon and he's CRUSHING IT SO FAR ... on pace to finish right around the 3-hour mark!!
43-year-old Johnson's been training for months with the goal of breaking the 3-hour mark .