Stinky - if it's any compensation the same thing happened to me towards the end of the RW half marathon programme. I was following the sub 01:50 pace, and early on in the programme I could do the intervals at the correct speed quite easily. In the last few weeks I just couldn't get anywhere near the speed I had at the beginning, let alone what I should have been doing according to the schedule. Maybe these training programmes just aren't always suitable for everyone, for whatever reason?
Val.