I would ditto about a pillow and stopping about every 2 hrs if possible. I am also dealing with degenerative back issues before RA so I found the two suggestions good.

Yes carry ice pack and lidocaine rub from Walmart it works great for long rides sitting down .goo luck try before you go on ride.

Hi Mary, have you tried a thin pillow ? Not sure but it might help give a bit of support. Just a thought.

I have terrible sciatica at times. Long car rides, or plane can be brutal. I have an obus form full back and seat cushion in car. And add a temperpedic " tush cush " if needed. You can get similar support cushions online.

Try using lidocaine patches they gel for 12 hours numb that area it helps on long drives