Any opinions on incline treadmill walking as part of a marathon training program?

Here are two sample workouts at this point in my training:
-I walk 145bpm heart rate, 6 miles at 20 minute/mile pace, 2 hours, 10degree incline
-I run 145 bpm heart rate, 8 miles at 12 minute/mile pace, 1.5 hours

For some reason my breath rate is faster for the incline workout even though my heart rate is the same. I like the incline treadmill because it is very little impact (i'm a beginner, 6'1, 200lbs), and I can fast forward to my favorite parts of bad movies on the DVR when I'm doing these long workouts.

So, any opinions on using these workouts as a minority component of a serious marathon training regimen?

http://This message has been edited by prepostrs (edited Aug-27-2007).