In one way, a day off is like a long run - it changes things up a little. One of the most common mistakes beginning runners make is getting into a rut; for example, running five miles every single day, rather than running a little more or a little less on any given day, but maintaining the average.
If you run 5,5,5,5,5,5,5 you never have a day any easier or harder than any other. By taking a day off and maintaining the average, you would run 6,6,6,0,6,6,6 (yes, I know that's an extra mile per week). The day off is obviously an easy day, but the following day will also seem easier, as one's rested. The day after that will seem difficult the first week after making the switch, because it's a 20% increase two days in a row. The next day may be even worse, but by the end of the first week, it'll seem "normal." The following weeks, the day after the day off will still seem easy and you may end up running it faster than usual - and that's where improvement begins.
Throwing a long day into the mix instead, one might try to run 5,5,0,5,5,10,5. The transition from 5 to 10 is too great to make in one leap and the variation is a little too great as well.
For the uber-geeky, I try to get the standard deviation of daily mileage to be 2/3rds of the mean.
I tend to think in minutes of training, rather than miles or kilometers, so a week's training for a marathon might be: 30, 90, 30, 90, 30, 165, 90. Training for the mile would look like: 20, 55, 20, 55, 20, 55, 75. These fit the standard deviation rule without having a day off. The long run for the mile is not difficult, but the one for the marathon would be a very hard run.
For my own training, I want to average 85 minutes per day in preparation for a trail 50K. The ideal week would be 80, 80, 80, 80, 0, 195, 80. There's no way I can just jump into runs over three hours, but I can run two hours, albeit slowly; I can't manage that for more than a couple of days in a row, however, which turns out to fit the math: 0, 120, 120, 120, 0, 120, 120.
The plan, then, would be "time on one's feet" training, trying to get used to running fairly long most days. As I get used to it, the number of miles I can do in that time will increase. When I no longer improve, it's time to start the next phase by extending one run into a long run while shortening the others.
Of course, once the temperature drops below 10 degrees, chances are I'll be slowing and thinking two hours is a ridiculously long time to run.
2 days ago