Official T&C is surely one breakfast per night. I suspect Hotel front desk would be confused if two restaurant/buffet charges posted to your bill. If at different times (6a and 10a) they woukd think one was misposted. Or, some restaurants go by a printout of whose rooms are inclusive of breakfast. If they crossed out your room # at 6am, they may not be too receptive when you show up at 10am.

Our Hyatt record has been 3 free breakfasts on a one night stay. Two separate restaurants first day (super early 7am checkin granted), and one restaurant the morning of checkout.

I suggest OP ask at the restaurant at 6a: Can I grab something quick now before my race and return for full breakfast later? Could always mention dietary restriction before the race but not after.