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Some venues do not allow refilling fuel unless it is at a designated place with mats under the bikes. That said, if no one sees that you have fuel in your pack and they don't see you fill up, no harm I guess. The venues where mats are required are usually on national forest or state land.

The things that will cost you positions are crashing, stalling and taking a wrong turn. Don't crash in the first turn, it's a long race. Definitely eat right and hydrate. If you aren't used to riding at a fast pace for 2 hours then take some ibuprofen before the race and possibly put an anti-cramping electrolyte in your water pack...