Tyler Cameron is living proof that no matter how in shape you might be ... a marathon can still kick your ass.

Gigi Hadid and Hannah Brown's ex-lover was among 50,000 people or so to run the New York City Marathon Sunday -- and probably among half who cramped up along the way. TMZ got this video of the model limping his way to one of the barricades in clear pain.

We're told this was shot about midway through the 26.2-mile trek throughout the city. Looks like Tyler's leg might've given out on him ... it happens, marathons are brutal.

Now here's the kicker ... Tyler ended up finishing the race, and in pretty good time too!!! We searched him on the official New York Road Runners' tracking system, and it looks like TC topped out at 4 hours, 39 minutes and 54 seconds -- an average of 10:40 minutes a mile.

Tyler was on a good 9-11 minute/mile pace until about mile 19, when he clocked in 13:39 -- and then dipping again at mile 22 at about 15:40. He picked it up at the end though, finishing mile 26 with an average speed of 11:08. Cramps be damned, he toughed it out.

Oddly enough, the guy hasn't posted a celebratory photo or vid of himself getting across the finish line ... even though he put up a before-photo. Modest much???