I am working backwards and trying to determine when to start each phase however I was wondering if I could get some ideas to the length of each phase - in weeks if possible
There are two “schools” of thinking in this regard: One group feels that you have to work in 6 weeks phases - the body can only adapt to the training, after a period of 6 weeks. The other group feels that 4 weeks are enough for adaptation.
I assume it will differ from athlete to athlete, but in general I found the following periods useful: GPP (6 weeks), SPP (firstly a 6 week period when you have split runs as preparation for special endurance, then 4 weeks, with Special Endurance in the programme), Pre-Comp (4 weeks). Motivation: Especially in an Endurance phase, you cannot expect the athlete to do e.g. Special Endurance - long, at full speed - for a period of 5 (hard) + 1 (recover) weeks. The fastest way to overtraining!
When I make use of the 4 week periods, I have few (if any) athletes with injuries and/or symptoms of overtraining.
Hope this information helps!