My four week program is really geared towards maintaining running fitness through a steady diet of standardized quality HIT workouts. It feels, in my opinion, less like a fitness regimen and more like a lifestyle-type commitment to high performance running. A four week program is perfect for someone who is already happy with where they are as a runner but wants to devote a minimum of time to maintaining their abilities, or someone who wants to get fitter but isn't really in a hurry to get there. That being said, depending on your motivation, it is possible to make good gains using this approach, you're just not 'forced' to as much--repeating efforts only once a month 'requires' less dramatic improvement over any fixed period between bigger efforts.

Four week program:

• Week 1:
  
  • Workout 1: Any speed interval workout (O1 or 2, I1 or 4)
  • Workout 2: Time trial, hills, double normal length--i.e. do it twice! (run as tempo run)
  • Workout 3: OFF
• Week 2:
  
  • Workout 1: Any hill interval workout (O3 or I2 or I3)
  • Workout 2: Time trial, flat.
  • Workout 3: Long run, 120 minutes minus workout time from first two weeks.
• Week 3:
  
  • Workout 1: Speed interval, different from week 1
  • Workout 2: Time trial flat, double length (run as tempo)
  • Workout 3: OFF
• Week 4:
  • Workout 1: Any hill interval workout, different from week 2
  • Workout 2: time trial, hills
  • Workout 3: long run, 120 minutes minus workout time from weeks 3 and 4

This program has enough intensity to keep you getting fitter or maintain a pretty darn high level of running fitness, while also being sustainable over a LONG, LONG time. Awesome.