[rojeee]

Simply because I find it gets me much better results. I know that to run a 15 minute 5km I need to be able to run that pace in training. I don't really care what my lactate threshold or VO2max is other than it's just interesting to know. I use a critical speed (pace/duration) model to know what paces are metabolically steady state and which are not. So based upon how I perform in training, I know pretty much what paces should be within the realms of possibility for a maximal effort and I use that to guide my training.

edit: I'll just add that I do use physiology to guide the training paces. But when you've been training for a while you know that 1) Marathon pace is aerobic threshold pace. 2) Half maraton is lactate threshold pace. 3) 5km is critical speed. 4) 10km is between critical speed and lactate threshold. So whilst I do use physiology to guide trianing paces I don't use a zone system or anything like that. I just design workouts which I know touch on the various systems I need to improve.