I've done a couple 6 hours now and did a 12 hour last Saturday followed up by a Cross race on Sunday.
For endurance, I think saddle time is important. I use my road bike for some long distance riding and mix in the summer mtb time trial series for some all out butt busting.
My 6 hour races have always been with a teammate but we decided after doing the 12 hour that we're going to do the 6 hour solo next year so instead of being teammates, we'll be fierce competitors.
If you've got some decent saddle time, you should be okay.
I'm not a solid food eater when it comes to racing. My stomach shuts down pretty quickly but I suggest doing what works for you.
Most importantly, have fun!
Streamers Jeni