During Base Building you develop cardiovascular and pulmonary function. This phase focuses on improving your ability to transport oxygen. The workouts during this phase are characterized by increasing the amount of time / distance you're running. Gradually increase your weekly mileage, and increase the distance of your long run. This phase should be the longest of your training. It should last at least 12 weeks and possibly up to 16 weeks.
The Base Building phase is all about building aerobic fitness. Run the same weekly mileage for at least 2 weeks before increasing your mileage. Only advanced runners should do a long run every week. All others should do one long run every 2 or 3 weeks.
You shouldn't race during the Base Building phase. If you choose to race, your effort shouldn't exceed 85% of maximum heart rate. You shouldn't take time off, either before or after a race during the Base Building phase.
Work in 4-week cycles. Week 1 is your "base" week. In week 2 increase your weekly mileage by about 10%. In week 3 increase the distance of your long run, but keep the total weekly mileage the same as week 2. Week 4 is your recovery week - return to "base" week mileage.
This plan gives you 4 long runs, from 20 to 30 miles during a 16- week period, which will prepare you to finish your first ultra marathon. If you're an experienced marathoner, you may increase that number by adding a long run on your "mileage" week - increase the long run that week to the same distance as your "long" week.
Very experienced runners may also add a back-to-back long weekend. Run a long run on Saturday and another long run on Sunday. (Don't attempt this unless you're an experienced runner who has finished many marathons.)
Goals Of The Base Building Phase:
· Build cardiovascular and muscular endurance.
· Increase aerobic fitness.
· Increase weekly mileage.
· Increase length of long run.
· Improve VO2 max.