Marking the Occasion

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 question!

• Spot on! The title refers to a (precisely) concurrent event though. :-) – Bass Jan 17 at 9:01
• Ha! @Bass you must have had this one up your sleeve for a little while, nervously watching that tag count! – Stiv Jan 17 at 11:59
• @Stiv Exactly the opposite, actually! I was cleaning up tags (on a pure formation-of-numbers question), when I noticed that I'd just removed the crucial tag, so I figured I had about half an hour to come up with a suitable riddle to claim it for myself. Checking my activity list, it took about 27 minutes; pure luck really, that nobody happened to post during that time :-) – Bass Jan 17 at 12:13
• @B540Glenn - Its shows my desktop i.stack.imgur.com/MuagE.png on clicking the tag but not on my mobile. – Tom Jan 17 at 14:46
• @tom Since your picture shows "sorted by active", which constitutes only an incomplete proof, here's mine from the time just after posting. – Bass Jan 17 at 15:06