Here's a photo of three gentlemen running in the first Olympic marathon in 1896 attired in what looks like street clothes.
This was the second modern running of the marathon; the first was a pre-Olympic qualifying race held a month before. In the Olympic race, seventeen competitors started the race and only about half finished. The winning time was just under three hours and the third place finisher was disqualified for covering "part of the course by carriage". I would also not be surprised if the three fellows in the photo above stopped off for a coffee and some painting along the way.