The idea of oversaturation (that does not solve itself and settle upon equilibrium) sort of reminds me of the concept of deer overpopulation. In reality it will not happen or else they will end up dying off on their own. The market will bear what it will. No way would I want to run a marathon in June, July, or August anywhere in the lower 48 (nor in Hawai'i) — but then I am not keen on running a marathon at altitude, either. As a rule, seasoned vets on the scene should be wise enough to avoid a race, especially a marathon, in its first year (or three) of existence. So, seemingly it would likely mostly be novices (including one-and-doners) filling out the fields for the new marathons for the first couple of years (should the races even make it beyond their first edition) as well as people who live in and around that town/county.