First let me ask have you been doing all the preventive maintenance on your vehicle. You know checking the coolant level on a regular basis and topped off or have you just used the vehicle and figured no need to check the coolant?
Also have you been keeping the coolant in the overflow bottle at the "full cold" line when the engine is cool or have you driven with the overflow tank empty?
I have read stuff from GM with vehicles using Dexcool and now GM recommends dealers fill the overflow bottle to the "full hot" instead of the "full cool" line when service is done on a cool engine.
If you have been running the overflow tank low or empty then you have had air in your system, that would explain in part why you have rust.
The drive you took was a good thing, flushed out some bad stuff.
I had a buddy whos family moved from SF, CA to St Louis, Mo in the early 80s. They drove their cars to missouri. He told me they were perking out rust and sludge out of the engines. They had a 65 chevy which they had added a coolant recovery bottle to. They would climb these big grades and the coolant in the recovery bottle was perking and there was a 1/2 to 2 inchs of sludge in the bottom. They had it happen several times but finally the car "cleaned" itself.
I take it your XJ does a lot of trips less than 30 minutes so it gets warm but not totally. I think you just gave your XJ a good run and if you travel again to that place check your coolant again you may of had more brown in the coolant.
Coolant does lose it's anti corrosion addatives over time. I know alot of people who change coolant every 2 years when using the green coolant.
I think part of the issue may of been you changed the way you drove the XJ. The engine is use to what every you normally do and then you put it on the road for 20 to 30 hours and it was running fully warmed up for 8-12 hours vs less than a hour.
Just brain storming.
Martin