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 mile 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 ... and at the time of this post, he's almost at the halfway point -- on pace for a 3:03 final time based on his splits.

What's even more amazing ... Jimmie's never completed a marathon before!

Oh, and on Saturday, he was RACING CARS -- he competed in the Toyota Owners 400, a 300-mile race at the Richmond Raceway in Virginia.

Jimmie is wearing bib #4848 during the marathon, a tribute to his #48 car.

Johnson says he's always wanted to run the marathon but his NASCAR schedule got in the way -- but this year, NASCAR shifted the schedule, which freed him up to run.

During his training, Johnson suffered injuries to his pelvis, left calf and IT band -- but battled through it ... and he's out there running right now!!

If you want to track Jimmie, click here.

GO JIMMIE GO!!!