It's good that you are riding. Now just do it more. Multiple hours. Lots of miles. Don't stop, slow down if you need to rest, but keep going. This will help you build better endurance and get in better shape.
Get another water bottle cage too. Drink before you are thirsty, eat before you are hungry. You need to stay hydrated and fueled, this will help a lot too.
Keep it fun, don't make riding like work and you'll keep wanting to do it.