I've done several "looped" centuries. They are all right ... and they are especially good for the support factor because you can go home at the end of every loop to restock your water, etc. ... but my problem with them is that I get bored.
In a situation like yours, I'd probably get bored after the 3rd loop ... and then I'd have to push myself out the door to keep riding ... and I wouldn't have fun anymore.
Can you find a loop that is a bit longer - perhaps 25 miles each? That way you'd only have to do 4 of them to get your century. Or can you scout out two or three loops in different directions from your home? You might do the one you've found for the first loop, then another one about the same distance, or perhaps a bit longer, in a different direction for the second loop, then either back to the first one again, or maybe onto a third loop in yet another direction ... or something like that.
I need variety when I do long rides ... if I am stuck on one loop, I'll ride it one direction the first time, and then the opposite direction the next time ... and I'll try to add little bits and pieces to it, like going around a block in a residential area halfway through the loop to add an extra mile or two ... or something to keep it from being the same thing over and over again.
If you've got a map of your city and the surrounding area, try riding out in different directions to see what's there on your training rides, and then shortly before the century, decide what roads you'll use from the many you've ridden building up to it.