We count the descent time in to the training total time. You’re right that they are different but the downs build a special kind of strength that you need in the mountains. Ultimately you need to just be consistent in how you count your training time. And, understand that the times given for the Z1-2 aerobic workouts is a sort of middle of the road guesstimate. If you have more time or more training background feel free to increase the volume. As long as you are recovering well more is probably better.