I was hoping to apply to most of my top choice schools early action, since most of them were large competitive public schools, where my application would probably be overlooked in the tens of thousands of regular action applications. I don’t have many extracurricular activities, but I was going to be able to say that I had run a marathon this year, but unfortunately I got injured a few weeks before the actual race. I have started retraining for another marathon, but it won’t take place until about a month after most early action deadlines. I know some schools say that you can call them to update your application, but I don’t know if that would actually help or not if apply early action, implying that I was not as ready as I thought. How does an extracurricular that says marathon training and intent to for a marathon compare to actually finishing a marathon? Would it be better to wait until the regular action deadline?
btw, this is pretty much my only extracurricular, so it’s probably important that they know about it.
You can always send an email to your admissions officer to update him or her.
Running a marathon is not a noteworthy enough EC to warrant sending an email to your regional admissions officer.
It is noteworthy if you do well. 
But then it might be the track or cross country coach that you would want to tell! In general, if you have a significant EC accomplishment, you can email the general admissions email and ask them to please consider the additional information as part of your application. I am often wary of emailing the regional officer directly – sometimes they switch regions or go on leave or even quit, and the website is not updated, so you can’t be sure it got to them or to the right person. If you send it to the general admissions email, they will make sure it gets to the right person.