Lopen = to walk. Sports contests are an exception, lopen is used in stead of rennen (also for sprint distances). I'm not sure why, maybe a short form of hardlopen.
Ahh op die fiets. I knew lopen was walk, that's why I was confused as to why it translated into the sentence as ran. Wasn't aware it was an exception in sports! Bedankt :)
Say you did actually walk your first marathon (which is still a non-trivial accomplishment), how would you say that?
While in progress: ik ben mijn eerste marathon aan het lopen.
When you've finished it (say last week): ik heb vorige week mijn eerste marathon gelopen.
"I have walked my first marathon" was not accepted, but i think it should, because it is the literal translation and it is gramatically correct. I reported it. Actually I remember a case where a marathon was organized and the temperature was quite high and some people fainted. The organisation told everybody to stop running and continue WALKING. Afterwards people could say : "I have walked my first marathon".