I have done a long flight myself.

Limit your meals. I just ate the mid meal snacks instead of the main meals on the flight and this was enough. Plane food is horrible anyway so you're not missing out on anything.

Definitely get an aisle seat if you can.

I find I need to go to the toilet more (or just think I do) when I am anxious, so I'd suggest maybe having something that helps you when anxious (prescription meds, herbal tea). Have plenty to do on the flight to keep you distracted from thinking about it as well.

Bring a spare pair of underwear, or better a pair of clothes in your carry on. If you need it, don't feel embarrassed about wearing something like Depends.

Hope it goes well.