A marathon = 42.195 kilometres (26.22 miles) in distance.
https://qph.fs.quoracdn.net/main-qimg-211694a3af29cf31d46cb8e9516a9396
It’s been stated in the comics, that he can run a mile over a minute which mean he can run at a speed of 60 mph (96.5606 kph).
The time would depend upon whether or not he was cheating by using Nike's carbon plated Vaporflys...
shareI thought he only had peak human strength and speed (although he appears much stronger than that in the movies).
Which would mean he could run a mile in about 3 minutes and 45 seconds, but could maintain that pace for the whole marathon thanks to his stamina.
That would mean he could run the whole marathon in about an hour and 38 minutes. (The current marathon record is just under 2 hours and 2 minutes, a pace of 4 minutes and 39 seconds per mile.)
Most people can run a mile in 8 minutes. People in good shape can run it 6. Athletes can run it in 5 minutes. But maintaining that pace for over 3 miles is tough.
In "The Winter Soldier", Sam tells him, "You just ran 13 miles in, like, 30 minutes."
13/30 x 2 = 26/60
So, MCU Cap would take approximately 60 minutes to run a marathon.