It's a good idea! Quit riding if you're having a miserable time! Bikes are fun, yo. If it turns out you're having a bad time due to endurance, the weather, mechanicals, or boredom, quit following the route and figure out a way home.
Look for short cuts on your route. On the 90 miler, turn left at mile 37.2 or 67.4. On the 60 miler/100km route (awesome route!), turn left at the 42.2 mark. On the 1/2 metric 37 miler, wiggle your way back to the start from the 17.5 point.
These are all examples of natural spots to make this your own ride. We want you to push through, as there are some great trails you will be missing, but take care of yourself out there!