Our team will phase in workouts over the summer, and have 2 "workouts" per week. So for example, the first week for someone working up to 80 miles per week is 50 miles for the week, with 1 hill session of 6x100m progressively getting faster (the hill shouldn't be too steep) and 12x200 at no faster than mile pace with 1:30-2:00 200 jog recovery.
Then a week at 65 miles, or halfway to 80, would include a 4-5 mile tempo one day, and something like 4-5x1200 at 10k pace w/3:00 recovery, and a long run with progression.
At your max, the tempo would bump up to 5-6 and the workout might be 5x1600 at 10k pace w/4:00 recovery.
We also do 10x10-15 sec. all out steep hill sprints once per week to help build muscular strength.
The single most important thing about these workouts is that you should never go faster than the assigned pace, even if it feels way to easy. Also, if you feel crappy, just skip the workout and do easy mileage. Any road race you do (our coach recommends 2 races from 5k-10k over the summer) takes the place of the harder workout that week.
Also make sure to adjust for heat, so if your tempo pace is 5:45 per mile, you might want to start of at 6:05 and you could cut down from there if it's really hot out. Essentially just run by feel.