I don't have any letters.
I used to have one letter (and a dash).
If you run, I have one letter (and a digit).
When I have twelve letters, no two of them are the same.
What am I?
I don't have any letters.
5000 - just numbers.
I used to have one letter (and a dash).
$\overline V$ - Roman numeral for 5000.
If you run, I have one letter (and a digit).
5k - a running event.
When I have twelve letters, no two of them are the same.
five thousand - the largest isogrammic number (no repeating letters).
Title - Marking the Occasion
This is the 5000th riddle question!