I have recurring lower back pain that I experience while driving/flying over periods of 30 minutes that leads me in pain for days. I use lumbar support and I sit quite upright. My doctor advised me to just pre-medicate with tylenol. Thoughts?

Cause. If it does respond to simple things like Tylenol and a corset that is certainly fine. If it does not respond, the bigger question is what is the underlying condition that is causing you to have recurrent problems with your back.
Comments. Due to your description, it seems imperative to me that you uncover the cause of your affliction. There are a number of structural problems which could be reversed by medications, injections, physical therapy, exercise, acupuncture, or even surgery. Seems unfortunate to live with this at age 27. Get a second opinion.
So the pain. is associated with prolonged sitting. A major cause would be degenerative disc disease, even in a person as young as yourself. Good posture, such as you are doing, is helpful, as is conditioning by exercise or walking. Taking OTC pain-killers prior to sitting is fine: you may find non-steroidal anti-inflammatories such as ibuprofen/naproxen more effective than acetaminophen/Tylenol. Good wishes:)