A year ago, I prepared a 9-day plan based on Pfitzinger half-marathon schedules from his 1999 book. My fitness improved dramatically and quickly; my 5K went from 20 minutes in November to 18:57 in April. Also in April, I ran my target half marathon in 1:28, which was a big leap from a 3:25 marathon the previous October.
After the half marathon, I adapted Pfitzinger's more recent marathon schedules to a nine-day cycle. Injuries largely derailed that plan, but I still managed a 59:59 15K and a 3:07 marathon as a 58-year-old. Perhaps a 10-day cycle with an additional recovery day would have helped prevent the injuries.
Nine-day half-marathon schedule:
Day 1: Long run 12-20 miles as progression
Day2: Off or bike
Day 3: VO2max repeats
Day 4: Easy recovery run
Day 5: Secondary long run 9-13 miles as progression
Day 6: Off, easy recovery or bike
Day 7: Easy recovery with strides
Day 8: Lactate Threshold repeats or 4-5 miles
Day 9: Easy recovery or bike
In hindsight, I would add dedicated strength training and stretching, and maybe not have ramped up overall mileage so quickly. My training the previous year was limited to only two or three days running per week.