Speed Building Phase
Only experienced ultra-marathoners should try a Speed Building phase. Even experienced ultra-marathoners that aren't attempting to "race" an ultra-marathon, including those that are looking to improve their time, would do well to skip the Speed building phase.
If you do use this phase, maintain your long runs, reduce your weekly mileage slightly and add time trials and races as speed work.
Conventional speed work is inappropriate for ultra-marathoners. Instead, on weeks you don't do a long run, run a race - 10K or longer - or do a time trial - again, 10K or longer. Your longest race or time trial, will depend on two factors. First is your ability to run a race at less than maximum effort. If you're capable of racing at 85-90% of maximum effort, races will help you. If you do them at a greater effort, they'll hurt you by limiting your ability to train after the race. The second factor is your ability to recover after a hard effort. If you need 2 or 3 days off after a race, don't race. You'll lose too much training time, and jeopardize your ultra marathon. To speed up your recovery and stay on the road after your hard efforts, pay close attention to NUTRITION AND FUELING.