Will sedans give me back pain/discomfort in long drive?

This is so dependent on you, your body type, fitness, age, height, proportions, interior space, seat, and preference. It's so insanely situational there's literally nothing any of us can say of value. Based on my recollection of comfort levels of vehicles I've driven regularly it'd go like this, 2012 Mazda 3 2015 Mazda 6 2018 Honda Civic 2004 Chevrolet Suburban 1988 Mazda MX-6 2022 Mercedes Sprinter 2008 Honda Element 2001 Honda Civic 2018 Toyota Corolla 2014 Honda Civic 2009 Chevy Silverado 2006 Mazda Miata 2019 Ford Transit 2006 Hyundai Sonata 2008 Hyundai Santa Fe 2023 Ford Transit 1996 Plymouth Breeze 2015 Honda Fit 2007 Chevy HHR 2010 Honda Fit 2004 Ford Focus 1997 Ford Escort 2003 Mazda Tribute 1988 Mazda B2200 The only ones I'd say I actually found comfortable, could drive for 12+ hours without pain, not just neutral or worse were the Mazda3 and Mazda6. I've rented and ridden in a ton of crossovers, pickups, and minivans. Personally for me and my body type I find compact or midsize sedans/hatches with firm seats, lots of bolstering, and a lot of lumbar support the most comfortable. But my friends who have gushed about their Buicks or Impalas, and they're comfortable for a half hour, but after that I'm in pain. I found my buddy's S10 Blazer way more comfortable than his Le Sabre which theoretically should have been more comfortable. There's no one size fits all answer. You just have to know your body and try different cars. The only trend you can discern is, generally, more expensive cars will have nicer seats.