Depends on what you mean, are we talking overall trips or single runs?
Longest overall trip was a little route through the midwest I took in Jan 2000 in my old Tbird. 10 days, something like 2500 miles from knoxville to des moines, up to minneapolis, over to indianapolis.
Longest single run is a long story
Ok... I bought a car online to use for parts, a 1985 Chrysler lebaron T1 (2.2 turbo engine is what I was after). Well, it was in Florida, I was in Tennessee. I was going to take a trip to see some friends in Atlanta one weekend anyways, and a friend of mine was in Miami with his massive truck, so we wrangled together a road trip out of it and I'd meet him in florida, rent a dolly, and haul it back to TN (he lived up here too).
Ok, I was driving a junker at the time... now, the above Tbird had 180,000 miles on it but was solid. This... this was a '91 shadow with almost 100K, but had been thrashed within inches of it's life by a previous owner. Flooded, too, so there's no interior panels, no carpet, just bare metal. No air conditioning either. Loud as heck but liveable.
halfway to atlanta louder got LOUD. The EGR exhaust crossover hose unbolted itself and fell off... meaning direct exhaust leak underhood. Whoops, I'm now driving with all 4 windows down the entire trip. Ok, no big deal, it's mild.
So the trip goes Atlanta, Zephyr hills, Knoxville. alright. We hook te car up and head on the highway and immediately I notice two things. One, bud's truck is geared to redline at 75mph. Two, i've got all my windows down and I'm following a truck with a hopped up cummins diesel spinning at right around 3800rpm. I think i got more exhaust from the truck than my own leak!
that was the longest THIRTY FOUR HOURS of my driving career