I would think that you would likely run out of gas before the start/finish. The more you are on the main jet(race pace), the more fuel you will consume. A larger tank will run you $165 or so and is easily changed.
Real world experience: I raced an Enduro that was axle deep mud for the entire loop. After 15 miles of trail, 40 mins down on the race pace, justql and I bailed and road back to the pit area on an asphalt road about 5 miles. When I arrived home, I drained the tank and only had 1/2 gallon of fuel. I used 1.3 gallons in 20 miles. Our speeds were slow but our engine revs were high due to the deep mud. justql's KTM overheated several times due to the slow speeds and high rpm's in the mud. Moral of the story, MX tanks are designed for MX not Cross Country or Enduro.