The rule I use for deciding when to begin adding more high intensity work into an athlete’s plan is the following.
When your aerobic threshold (as measured either by pace or heart rate) is within 10% of your anaerobic threshold (also as measured by pace or HR).
After 15 months you have hopefully seen a steady increase in the speed you can run, bike, hike at AeT and also the AeT HR has climbed. If it has plateaued and not improved after several consecutive weeks of training that is another sign that it is time to add high intensity.
So how do you find you anaerobic threshold? Easy: Do a 30 minute time trial using the same mode of training as you do most of aerobic workouts. If that is running then the time trial will be running. If it is hiking then it should be an uphill hike. This test should look like this: 15 minutes warm up so that you have broken a sweat and you HR get above the AeT for at least 2-3 minutes. The start the time trial. It might be a 30 minute uphill hike or run. About 10 minutes after starting the time trial begin recording your HR with your nice GPS/HR monitor watch. If you do not have a recording HR monitor then just keep an eye on where your HR settles in for the remaining 20 minutes of the time trial. The effort level needs to be as hard as you can go for the whole 30 minutes. Of course this means pacing yourself so that you do not go out so hard you fade badly in the last 10 minutes. So this should feel like a race. At the end of this test you will see what the average HR you maintained for the last 20 minutes. The reason for not including the HR from the first 10 minutes is that most people will go out too hard and it will take them 10 minutes to settle into a manageable pace. So the HR from those first 10 minutes will probably be too high. The average HR for this last 20 minutes is a good approximation of your AnT HR. If you are an extremely well trained endurance athlete then you will need to make this a one hour time trial but for most folks 30-40 minutes will give good value.
So lets say that your AeT HR is currently at 155 and when you do this 30 minute AnT test you find that you can maintain about 170 for over the course of the last 20 minutes. The difference between these is 15 beats/minute which would be less that 10%. If this is the case with you then by all means begin to add some high intensity training in the form of steady paced workouts at AnT or intervals done at above AnT HR. Do this 1-2 times/week and you will really see some big gains in endurance performance. Be sure to maintain a good volume of sub AeT work though as it is needed to support the high intensity stuff.
By the way, do not fall into the trap of trying to target training of a specific physiologic measure like maxVO2. You will be better off to training to improve your endurance performance. There is no prize given for the athlete with the highest maxVO2. The races go to the fastest runner not the one who does the best in a lab test. Outdoor time trials will be a much better metric of endurance improvement.
Look for an article on this soon.