20 miles??? That's nothing, even for me, and I am over 55 and overweight. I ride that far almost every day (it's 9 miles to the nearest town).
Ride some centuries. Start at 50 miles, and work up to 100, 200, and even 300-mile multi-day trips. Then, you are doing long-distance cycling.
You need to be self-sufficient on the road. Make sure you have a good multi-tool kit, tire repair kit, a pair of pliers, and an adjustable wrench. With these tools, you can fix anything that will go wrong, that can be fixed on the road. Learn how to do basic road repairs. Always check your bike thoroughly before you leave.
Always have your cell phone, charged fully, and bring the charging cable with you to recharge the phone at restaurants, etc.... Bike computers are nice, but they don't do anything you can't do with maps, a wristwatch, and the ability to do math. I did long-distance a long time before they had bike computers.
I'd keep the equipment to a minimum. A credit card weighs a lot less. and hostels and motel rooms are easier than lugging a tent and camping gear, unless you actually are going to camp somewhere cool, like a lake, river, etc.... Just have a change of clothes, minimum toiletries, and let someone know where you are going and what route you are taking, before you leave.
The most important thing to take is water. At least 2 bottles, 4 is even better. It can be a long time between refills in some areas.
Most of all...have fun. It's not just a ride, it's an adventure.