The answer to your question would be that the past participle of the verb "to run" is the following one: Run. That is, the sentence would be as follows: Jaffa run in more than a dozen marathons. However, you need the auxiliary verb to indicate that the marathons have taken place over the course of Jaffa's life. Therefore, the sentence would be "Jaffa has run in more than a dozen marathons". This sentence is in the present perfect tense.