Yes! You can travel long distances with your cat on a harness. It can be done. Two years ago I moved from Dallas Tx back home to Vancouver, Wa. A month before the trip I bought a harness & leash and started dragging it around, and let my cat bat at it and get used to it. Daily I started putting the harness part on her then toss her treatsm then I fed her dinner to her while she was wearing it. Initially she just laid there like a 100 ton weight had been applied and snapped her precious legs off. Then, she started moving around cautiously until it became second nature to have it on. That is, after she was satisfied that rolling around and rubbing it on things that it wasn't going to come off anytime soon. After awhile I was able to leave the leash on her to drag around and she accepted it as well as any cat could. I started picking up the leash and walking with her on it. It was like it was her idea. The trip home was great - she was able to exit the car with me and we stopped in many national parks (The Grand Canyon and Painted Desert were spectacular!) and I was able to get her into the fresh air. She especially liked a park in New Mexico that was along some water. She found the huge bugs fascinating. She used the litter box (it was in the back of the blazer) during the drive and slept and meowed like normal.
I wanted to share this story because putting a leash on a cat can be done with a little bit of patience and care. Good luck to all that travel!