How to tackle a long ride when you're having a bad day.

Plan A: Go with a group. When you've got to tackle a long day in the saddle it's always a good idea to go with a group, especially if you're having a bad day and lacking for a little motivation. A group provides a bit of peer pressure and gives you some company to push though the long hours in the saddle.
That is a good idea unless you're having a real bad day and get dropped on the first climb 45 minutes in to the ride. Note getting dropped can be ok especially if it provides you the motivation keep working hard so that you ultimately get better. However, it can backfire too if you get demotivated by getting dropped by the 60+ riders in their wool shorts, hairy legs, and circa 1982 bikes with downtube shifters.

So if Plan A fails, it's on to plan B for surving a long ride when you're having a bad day.

Plan B
1. Go out on a big loop so that you get to a point of no return before you completely lose your will to go on, and then are forced to ride home to finish the day. Best to find a loop with no short cuts home.
2. Find roads out in the middle of nowhere with no houses, towns, or other signs of civilization to avoid the temptation of stopping and calling for a ride home.
3. Don't bring your cell phone for the same reason, you may be tempted to call for a ride home when you are still several hours away from your goal.
4. Have a mid-way destination or goal to help you push through the first half of the ride.
5. On that mid-way stop have a little caffeine and a tasty snack. The caffeine may provide a little pick-me-up (for about 30 minutes), but the tasty snack if laden with lots of fat and sugar will provide enough guilt for you to finish the last few hours of riding, especially if said snack is not part of your "training diet".

6. Time the end of the ride with the weather so that you are forced to keep riding hard in the last hour to beat the incoming storm.
7. If you are still short on your time goal despite how slow you rode, just do laps around the neighborhood just so you can say you finished.