The short answer is, no, from my understanding you can't include walk breaks and reap the same benefits from the slow workout. You can go slower than the prescribed pace, but you should attempt to not go faster than the pace prescribed.
But, as a scientist, I always like to break down the why to any of my answers:
The average person walks a 20 minute/mile (3 mph) pace. To use Shannon's plan as an example,
On 1/17/16, her training plan states she is to run 8 miles at her "Long Run" pace (11:05 min/mile). The "Long Run" pace is at the upper end of the aerobic training zone (slow easy pace). While it is a continuum where neither aerobic or anaerobic is being used exclusively, faster than the Long Run pace moves away from the Easy (slow) running and into the Hard category.
Let's say she wanted to break up every mile with 1.5 minutes of walking. At a 20 min/mile pace, she will have covered 0.075 miles in 1.5 minutes. This means she now needs to cover 0.925 miles in the remaining 9:35 minutes (to reach a 11:05 min/mie average). This means to cover 0.925 miles in 9:35 she would need to run a pace of a 10:22 min/mile. So truly, 92.5% of the mile was now at 10:22 min/mile and 7.5% of the mile was at 20 min/mile. This 10:22 min/mile is right at her marathon pace. This makes what was an "Easy" 11:05 min/mile pace at the upper threshold of aerobic (endurance) training to now into a "Hard" workout closer to a marathon tempo workout.
Let's take another hypothetical example from my training,
Workout A - 7.25 miles at a pace of 9:21 min/mile
Workout B - 7.25 miles at a pace of 9:21 min/mile
From the overview these appear to be exactly the same, but this is what I did for Workout A:
I did a 6 x 800m workout. Each interval was at a 7:13 min/mile which is about my 5K pace. Adding in warm-up, cool-down, and resting intervals I covered 7.25 miles at an average pace of 9:21 min/mile. However, 3 miles of the 7.25 was done at 7:13 min/mile, and the other 4.25 miles at ~10:33 min/mile. This is considered a Hard workout.
And Workout B:
7.25 miles at a steady 9:21 min/mile. This is considered an Easy workout.
The benefits received from each of these workouts is different. Workout A works on running efficiency, breathing, turnover, and running on fatigued legs. Workout B is an easy workout meant to build all of the benefits discussed earlier about slow running.
Does this help?