Given the size of the battery (and gas tank, should you choose the REx), you'll have to be stopping fairly often to stretch out. Personally, I've not had any issues with mine, and I do have a bad back. If you want to drive it for hours on end, IMHO, you've chosen the wrong car!
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)