Yes, you can do it, just make sure you stay hydrated.... and it's helpful to keep some plain water with you in case you get too hot to splash on yourself to help cool off.

If you can plan the route with a rest stop (either a loop ending at your house you can repeat, or a longer route with a convenience store, etc) that's even better so you can make a potty break and refill bottles as needed. I have a 5.1 mile route from my house I repeat 2-3 times pretty regularly for 10 or 15 miles. It only has a 62' change in elevation with most of that as a gradual climb at the beginning, so for around here that's about as "flat" as it gets and it's the easiest one to do if I am going for distance. I haven't yet repeated it 4 times, but perhaps I'll go for it this weekend.

You're not crazy to try it.... I read on some website recently that anyone in fair shape can easily ride 20 miles.. the trick is though, if your rear isn't used to that amount of saddle time, or you have issues with bike fit, that is what will be problematic - it's not so much a question of endurance, but comfort.