I've done 5 marathons in the past year and half. I actually ran the pace you mentioned at Boston (6:25's)in my 2nd one.
My usualy week consisted of
1 long run at a solid pace that I could feel comfortable with for the majority of the run. Sure on the 18+ days I would be tired at the end, but the majority of the run I felt smooth and in control. (I would usually try and run my desired marathon pace for as long as I could).
1 workout that made me push myself. 6 X Mile repeats @ 5:15-20, 10-15 fartlecks, 3:00 intervals with a nasty hill in the middle, out & back faster (way back try to make the course uphill), in & outs, etc...
the rest I would fill with varying BPMs and easy days depending on how I felt. My BPMs were at a level where I could talk to someone but not at full length, easy days I could sing a song while running.
Every week I would increase my long run and difficulty of the the workout slightly as to avoid just jumping into long runs I wasnt ready to handle or workouts that left me too tired to run effectivly.
I'm not what you would call a really 'good' runner, but all of my marathons have been under 3 hours so I'm not horrible either. I'm sure you can get some better advice, but I thought I'd share my 2 cents.