I have taken a few long distance rides on my Custom. As other's have suggested, way too many for one day, especially in the stock seat. Split it up over a few days, or more if you have the time and resources.
If you have saddlebags (or any type of storage, such as a backpack) keep a few tools handy (you never know), and pack for a few different types of weather. (I have been in some of the worst rain storms you could imagine on mine, and luckily had rain gear!) Stay hydrated, take frequent stops to stretch out. Again, as others have said, don't push it.
Check tire pressures, oil level, make sure everything is functioning before your ride, and enjoy! The Custom is a very competent cruiser for trips like this. (Although I couldn't ride longer than 30 minutes without pain, and ditched the stock seat long ago, others have found it fine for longer trips).