I'm no elite, but I run half marathons so take this for whatever it is worth:
Hydration: try one of the low or no sugar options. I've had good luck with Powerade Zero, G2, and Gu Brew. You can also opt for water only if you use something like Salt Stick or S-caps.
Really, as long as you are adequately healed and able to meet your fueling and hydration needs with your remodeled guts, then there is no reason to hold yourself back. Good luck to you!