The Approach: Higher Volume
No magic here. If you can increase your miles over time and absorb that load, you will improve. For many triathletes barely reaching weekly double digits, higher volume can mean increasing from miles in the 15–20-per-week range to the 30–40-a-week range.
The benefit of focusing on volume is that it’s easy to track and proven to work. Avoid the pitfall of increasing miles too quickly (fatigue and/or injury), and be sure to keep a balance of training between sports. Many triathletes eventually find a point of diminishing returns with run mileage, where it makes more sense to rest with additional time or where the recovery required from additional runs cuts too much into the energy required for quality cycling.
How to: Consider devoting a chunk of time solely to running (such as early spring if your main races are in the fall) where you’re willing and able to scale back on other sports. Keep a small amount of cycling and swimming in the plan, while keeping the recovery cost from those sports low. Just pick a simple metric to track (i.e. time or miles), and focus on increasing weekly training at a rate no greater than 10 percent a week.