StephenH

On where to ride- we can't help with that without knowing your area, which we likely don't.
But check what you can find for nearby bike shops and for nearby bike clubs, see if you can find out where they ride.
I believe on Strava (which I don't use), they have features showing high-ridership routes.
"In the city" can vary a lot, and there are places to ride in downtown Dallas, Fort Worth, etc. Maybe your area is especially sucky, so hard to say without knowing where that is.

On general strategy, go ride two or three times a week, 20-30 miles, work your way up to longer rides.
If you're young and athletic, it probably won't be that hard to do a century.
If you're old and fat, it's more of a challenge.
If you're serious about training, look into that, but be aware that a lot of the training is relating to racing, not to just riding around. So yeah, to perform your best, maybe you ought to sit in your garage for months on some computerized torture device. If you want to have fun, I'd advise a different route, though.
It is actually possible to ride a 100 miles (and longer) eating regular food. So you can get into all the nutritional drinks and stuff, but that's not a requirement.

I assume by HNH, you mean "Hotter'nHell" in Wichita Falls, Texas? Of if you mean some other event, clue us in.
It's a fairly flat course usually with lots of wind. Temperatures vary. I've done it three times, and it wasn't just super-hot any of those three times. But ride in the wind, ride in the heat, and it won't be a lot different.
The first two times I rode it, I was on my Worksman cruiser and took about 9.5 hours, so the bar is not real high. I had done a couple of 100k rides prior to that.
If you try to go fast, you'll have 10,000 people in the way, so to do a faster century, do a different location, or do that route at a different time.