After a fair bit of research I have decided to devote the majority of my training time, in the initial months at least, to developing my aerobic base. This means concentrating on exercises that improve the efficiency of type I, slow contraction, muscle fibres rather than working the type II, faster contraction, muscle fibres. This means mostly low-intensity running, swimming and aerobic machines at the gym.
As can be seen in this table, type I muscle fibres have an important property for endurance events: their maximum duration-of-use is measured in hours not minutes. The other great thing about these fibres is that their energy source is fats (triglycerides), not sugars; so training that primarily utilises these fibres should maximise fat loss.
Training type I fibres means staying below the anaerobic threshold (also called the lactate threshold), which is the point where lactic acid is produced faster than it can be removed. To estimate anaerobic threshold (AT) I am using the "180 formula" derived by Dr. Philip Maffetone.
By using a heart rate monitor during training I can ensure that I am always just underneath my AT. It is my AT level that sets my pace -- as soon as my heart monitor says "over zone" through my headphones, I drop back to a fast walk (I am using a Wahoo Bluetooth heart rate monitor in conduction with the iSmoothRun iPhone app). Each week, as my aerobic system improves, I should see my pace increase. The point when it stops improving will be the time to change to a different plan.
A lot more about this method of training can be found on the Natural Running Centre website.