A few comments from my peanut gallery.
10Ks, Halfs and full marathons... YAWN. Try Ironman competitions. Yes. I do all of these.
I have shot both Olympic Distance and Half Ironman distance races and the reason the photos are so much is the pass through of actual purchase is little. There are several class of people who do these races, the "bucket-listers" who are looking to finish, the "weekend warriors" who are penny pinching to do as many races as possible, and then the pro/semi-pro who likes to have photos for FB, sponsors, etc.
Shooting these events are not easy, especially crowded running races.
People want INDIVIDUAL shots. they are not looking for a group pic, unless they are next to someone famous, so if the course is looped and a pro racer or someone notable is passing a bucket-lister, then you might have a shot where they are "Cool.... look at me running with X"
Access to the courses is not always simple. In most cases you are dealing with blocked roads, crowds and once you get your spot, you are often stuck in your spot for a long time, and to get your best shot, you often have to "encroach" on the course, so not having credentials can create issues, as well as all the iPhoners who jump in your way. You have credentials you can tell them to back away since you have race permission and are actually part of the course, as well as for me, I can whip around the actual course on my motorcycle and get to different spots.
Shooting is not always easy or fun. I have been stuck at the side of the road for 4 hours and the race comes at you in feast and famine, and again, when you have 200 runners coming by you over a couple of minutes, trying to get "nice" shots can be a challenge, people are always coming in and out of frame and most of the time people look like they are about to take a dump, have a horrible expression on their face, or are blowing out a snot-rocket that looks like it is two tablespoons worth of tarter sauce (actual experience of a very well known female triathlete and when I saw it unfolding deliberately pointed the camera down to give her peace and space... respect goes a long way)
While it is true, most athletes are psyched to get a great pic, most are blase and some of the best folks out there that will shoot well could care less about another pic.
I think three or four of my shots are still currently FB avatars for some of the pros that I shot this last year, but I gave them those shots because I consider them friends, love seeing them out there and they are helping me as much as I am helping them.
Not saying there is not money to be made, but you would probably be more profitable shooting Senior Portraits off a listing on Craigslist
I am working it the opposite... trying to work with the lower level pros to build their image, license the race images to their sponsors, either for cash, or also works for me, gear (as in athletic gear, not camera gear)