Having discovered your AT, you need to work to improve it. Work around 5% below your known threshold, aiming to keep your heart rate there without ever actually going over your AT.
Start with one or two 5 to 6 minute repetitions, adding a minute per week until you can perform two ten minute intervals. Allow five minutes easy recovery between intervals for your heart rate to recover.
Once you can perform two 10 minute intervals, you can either add more intervals (for middle distance events) or more time (for long distance athletes). For example, for marathon training I perform two or three 20 minute intervals.
An alternative version of this is a timed-hill interval. For this you will want to use a hill that takes about 5 to 7 minutes to run or ride at your threshold heart rate. Complete the hill, and record your time and final heart rate. The goal is to then to try and lower your time without going over your threshold heart rate during the interval.
These sessions should be done once or twice a week in the build up to the race season, maybe once every other week during the off season. They should always be preceded and followed by an easy (recovery) day.
These are great workouts to mix in with your regular hill repeats, speed work and long distance workouts, ensuring that every workout is contributing to your overall progress.