Interesting read, since many of you see my posts and know I am training for a full marathon. And I picked a very structered and (to me) agressive training plan. I run 5 out of 7 days. I had a bad 7 miler on Sunday. (How bad can 7 miles be??)
Not only is this my first marathon, but I have set a lofty time goal.
That being said, I may re-think that.
My decision to go very structured is that is what I felt I needed. Too often I think I'm to tired or I'll run tomorrow or blah blah blah. (Skip workouts) So I wanted to push myself.
That being said...week 3 of running 5/7 days and I am tried. I am so glad NOT to put the running shoes on today (my rest day).

So I think it depends on your goals.
How motivated you are WITHOUT a very structured plan.
And over all fitness.

K