Hi Crow,
99.9% of fork seal leaks happen over time, not in a moment. So it is almost completely impossible that your seal blew because you drove on a bumpy road.
What most people don't realize is that long before you see any oil on the tube, the main seal will have been leaking under the dust seal, and the dust seal will wipe the tube clean (for the most part) up until the point the whole space underneath it is full of oil, and then it will begin to ooze over the dust seal. This can take thousands of miles sometimes.
Fork seals are pieces of rubber sliding against metal. They simply wear out. Some go 50,000 miles, some go 5,000 miles.
You will never get the same service interval out of a rebuild, because the fork tubes will not be in the same condition as when they were new. The will have pitting and scratches in them from normal use.
The entire left fork and brake assembly are an engineering joke. When you apply your brakes, the left fork is hydraulically locked by the anti-dive. This causes the entire wheel and axle assembly to **** against the fork tubes. The rotating left caliper induces an unnecessary load into the leg as well.
Disabling the anti-dive is the single cheapest and one of the biggest improvements to ride quality you can do to a GL1800. The bike will not "dive" if you do this... it has no where to dive to... it is almost completely bottomed out when stock. Roughly 60% of the travel is lost in "sag" before you even get on the bike.
If possible have someone polish your fork tubes on a lathe using a fine (400 grit) emory cloth to remove pits and scratches.
Then fill the sliding lip of the seal with a proper seal grease. If you don't grease the seals, they will wear out much faster, and you will have more drag. (you may notice some white slimy stuff in your seals when you take the forks apart... this is the factory grease from Showa...).