This is the 6688th most frequent Japanese word.
ハーフマラソン
Half marathon.
Here, 'ハーフマラソン' is used as the event itself 'half marathon'.
ハーフマラソンに参加する予定です。
I'm planning to participate in a half marathon.
In this sentence, 'ハーフマラソン' refers to the event that the subject ('friend') completed.
友達がハーフマラソンを走りきった。
My friend finished running a half marathon.
Here, 'ハーフマラソン' relates to the activity for which training is being done, representing the word's context of preparation or activity scope.