Although our results did point toward amount being more important, it is very likely fitness levels can be improved by increasing either amount or intensity.
Exercise amount appears to be more important than exercise intensity for eliciting gains in cardiovascular fitness.
Anything beyond walking briskly for 12 miles per week, whether increasing your intensity or the amount of miles, has additional benefits. So there is a separate and combined effect.
The classic question always is: What's the minimum amount I need to do to enjoy the benefits of it. If you just walk 12 miles a week at a brisk pace, it's scientifically proven now that you will get some benefits.