Runners should integrate into their training program lots of shorter distance racing. This would apply not only to marathon runners for whom short races are an excellent way of doing speed training, and of testing how their fitness is improving without having to run marathons and the like to prove themselves; this would also apply to middle distance runners; for example 800 metre specialists should race as many 200s and 400s as they can leading up to their main races without easing off their training.
The sample training sessions I have listed should not all be attempted at the same time. Instead they are part of a sensible 12 month pattern.
Here is a rough guide to assembling them into your year. Canberra middle distance Vets want to peak in March for the ACT championships and March/April for the nationals at Easter. (This year the ACT championships are at the end of February, because of the Commnwealth Games, but think ahead for the next twelve months).
January race practice & speed development the main focus
February race practice & anaerobic
April general endurance
May general endurance
June general endurance
July general endurance, hill sprints
August specific endurance, hill sprints
September specific endurance
October aerobic intervals
November aerobic intervals
December aerobic intervals
So that is how to train for an 800 metre pb in March. Remembering to ease back the distance during the racing period. And being confident that you will improve, having subjected yourself to the discipline of long and hard sessions through the 12 months, and being fortunate or clever enough to avoid injury through that period, keeping motivated through varying the training, doing additional cross training and jogging, and being part of a training group whose members are similarly motivated.