Improving a Treadmill Workout

Question:
When walking on a treadmill, which is better: increasing the time or the incline?

Answer:

Both are helpful. And that's what interval training is all about: Changing the level of intensity both by speed (faster and slower), as well as changing the incline. Both are great. Mix it up — that's the best way. Both work, and combining the two in a workout is the best!