It's not a hard thing to do, just ride. Start out with what you can do, get comfortable, and then add 5 miles or so ( or what you can push yourself to do). Keep working that progression until you hit 40 miles, then pick the day, and go.

For the big day, pick a flatter route, bring some good ride snacks, drink lots of fluids, nail your half, and revel in your success.

Too many people try to over think something like think this. It's riding a bike, something we've all learned to do when we were kids.