75 does seem a bit much. Our local groups tend to top out at 30 or so.
Riding single file would be pointless. There are a few spots where you might be able to pass if they're clustered but you could never do it if they were single file. Cars have to get into the opposing lane to pass and you would never have enough room before a turn or hill.
If you can't get past them, you wait until they get to a main road. Not more than 3 to 4 miles at most and usually less. Doesn't really delay a motorist that much. A group of 75 wouldn't be much different in that regard.
I ride solo myself, but the groups always pass my house as I live in an area with great roads for biking.