My marathon goal was to run around 2.30-35 (just under 6 min/mile) and I tried 2 different approaches to MP tempo runs. In both cases, I (attempted to) run a tempo at MP once a week, mostly on a Friday (long run Sunday, track workout Tuesday), which I guess is pretty standard 3 workouts/week supplemented with as much mileage as you can do without breaking down or leaving you tired for workouts:
1. Starting at 8 miles, increase 2 miles a week until I got to 18 miles, all ran at the same pace (MP, 6 min/mile approx). I ended up running 8, 10, 12, half marathon race, 15 and 18. The last 2 were a big struggle running alone for 90+min at MP. My marathon was 2.39
2. 10 mile tempo every week at approx 160 bpm heart rate. I did 5 of these, a half marathon and a 30km race in the build up. I didn't see a lot of progress in the tempos (expected to run quicker at the same HR the more of them I did, but the weather conditions and training fatigue seemed to have a bigger impact on pace). DNF in my marathon
I got to the 160 bpm heart rate by running 10km on the track at 6 min/mile and my average HR was 159.
In my first approach, I ran on a gravel trail. In my 2nd, I ran on a tarmac path. Both flat. I felt I needed to prepare my legs for the pounding on a harder surface after I slowed in the last 5km of my 2.39 marathon, but this may have resulted in my DNF, due to slight injuries.
I don't know what conclusions to draw and what approach I'd take if I ran (or coached) another marathon. With the progression of distance, I had the better result (I didn't dnf) but I felt the first 1-2 weeks were easy and the last 2 weeks i struggled to hold the pace which knocked my confidence going into the marathon.
In the 2nd approach, I tried to run the last 30 min of my long runs (2 hours) at MP, to supplement the tempo work. I found this tough but also thought it was beneficial.
One main learning was, try to find training partners to help you out on tempos or long runs. People argue running on your own makes you mentally tough for the race, but I found the pace is slower, so conclude the physiological benefit of running a faster workout in a group, outweighs any psychological benefit you might gain from solo runs.
I also probably obsessed a bit too much on pace and HR, trying to control things like a scientific experiment, because I was training alone. I almost forgot to enjoy running and certainly lost a competitive edge of racing training partners, as I was training to control my efforts all the time