I asked a guy at work who's pretty serious about his cycling what he thought, and he suggested illiotibial band syndrome. After reading the typical symptoms, it seems like that's exactly what I had.
I was really concerned by this, given it's so early in the training and the rest periods from here on are short. I decided not to play squash to give it more rest, and did as much stretching of it as possible. I also iced the area a couple of days.
So week one was a mixed bag - my target was 30k's, of which I only did 17.
So not the ideal start, but still 15 weeks to go, no need to panic just yet.