5 or 6 hours on your inner forearm is no walk in the park. That's some pretty thin, sensitive skin. For a long session I usually take 3 Advil before I start, 3 four hours later and then 3 more four hours after that. If you are going to sit that long I'd bring some Gatorade, power bars, banana, etc. Sip and graze, it cuts down on the bathroom breaks. I'd keep the breaks to a minimum. Every time you stop, you have to try to get back into the zone and it gets harder every time.
Good luck.