Help!: Hitting the Wall

I have been training for several years now, slowly building up from a 5K to a 10K and 1/2 marathon.  I ran a couple of marathons several years ago and my future goal is to try and qualify for Boston.  My question is this.

I have the mileage in, hill workouts, tempo runs and speed work in.  My problem is I am about 15 minutes away from my qualifying time of 3:30 or 8 minute miles.  I can't seem to get over the hump.  Any ideas on getting to my goal?

A bit more information might help.  Are you training with a HR monitor?  If so, are you spending reasonable amounts of time with your heart rate in zone 1-zone 2 (lower, aerobic zones)?  If you are doing hills, tempo runs, and speedwork without building your aerobic engine, a change of pace is needed.

 If you are doing reasonable aerobic work with weekly quality runs, then it's a matter of giving it time.  I've been training for over six years, and am only now getting close (my PR is still 30 minutes too slow) to a Boston qualifying time.  However, I spent the first four years doing all the wrong things.

I've spent the past 18 months focusing on improving my aerobic base and doing limited speedwork throughout most the year.  In the 8-16 weeks leading up to major races, I add more speedwork.