I've used Jack Daniels' mixed long runs (easy pace + threshold + easy pace + threshold) as part of marathon preparations. As he points out, it is a tough workout that approximates the kind of duress you might feel in the late stages of a marathon race. That said, it did little to raise my anaerobic threshold. It really is a long, endurance workout. By folding the threshold pace stuff into a long run, you are no longer getting threshold benefits, but rather the improvements associated with a raise in aerobic, or marathon, threshold. In short, it didn't make me faster, just tougher/more pace consistent.
For my most recent marathon, NYC 2006, I adopted Canova's methods. Two key workouts were hard uphill running of 100m with full rests. At most, I'd do 3 sets of 10 reps. The other hard quality day would include a 40 minute progression run, after a 20. min easy jog/warmup, that ended above threshold pace. It was challenging and left me quite winded. I felt I got quite a bit of bang for the buck, though. The important thing is the stuff at, or around, threshold pace was not part of a long run. That mix was too much and diminished the effects of the threshold/quality.
My specific marathon prep work revolved around long runs every 10-14 days where I started at a comfortably hard pace, with the last 40-60 minutes at marathon pace effort (using an HR monitor) for 2.5 - 3 hours total. This workout was not unlike the Daniels mixed long runs in feeling. Other workouts included 40-50 minutes at marathon pace.
The key thing is that I did no VO2max intervals - my hill repeats and weekly 40 progression/threshold run replaced those. These results were a six minute marathon PR over a tougher course (Chicago vs. NYC).