If you have a few and under your belt, you're probably thinking about running your first half marathon. Doubling your racing distance means building endurance by increasing your weekly mileage. Don't be daunted by this! Try this training
The Rookie Marathon Training Plan - this one looks pretty good but I'd recommend trying to go up to 22-23 miles before the big day - 20miles is typically "the wall" and if you can prove to yourself you can get past that BEFORE the race then all you have left is a 5k. That's my theory anyway.