The most common and most likely answer to this crossword clue is the 15 letter word INÂ THEÂ LONGÂ RUN
I've seen this crossword clue in The Guardian
I
N
Â
T
H
E
Â
L
O
N
G
Â
R
U
N
Buddy explains!
The straight part
We know that "Where a marathon runner may eventually succeed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where a marathon runner may eventually succeed = in the long run.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.