The HHH can be sort of variable. It's late enough in the summer that you're often past the peak heat, and it can be rougher riding in late July. And, it does have rest stops every 10 miles. It was the first century I did, and that was on a Worksman cruiser bike.

In addition to the organized centuries, you might also consider the randonneuring rides. Here in the Dallas area, we're normally riding something each weekend. Watch those rides, and get into one of the flatter ones and go for it. If you haven't seen anything prior, there's a brevet on December 20th that is reasonably flat. Watch the weather is the main thing there, strong north winds (or cold or wet) could make it a challenge, but if it's actually decent weather, it'd be a good ride. You'd probably want lights for that, due to the shorter days.