Hi. Thanks for the answers. I had done a 15 min warm up on the bike before, and weights before I got on the cross trainer....BUT I did exactly the same the day before, so yeah, maybe it was that my muscles hadn't recovered...??
I also had almost exactly the same to eat between the 2 days, as it was leftover day, so had exactly the same yesterday as I had the day before ha ha. Anyway, it was annoying as I was getting so tired....and not going today, I really wanted to do 40 mins (I did....I made myself finish it, but it was a monumentous effort!)