Thanks for the comments people.
I tried riding to work today (about 18km) at a higher cadence. I was having trouble trying to keep in the 90s, my legs tended to go like thrashing machines! I was more comfortable in the high 80s.
On longer randonneur type rides I spin more (faster) and I find I don't tire as easy. On the weekend I did a ride of about 110km and tended to push the bigger gears (staying mostly in the 70s) and found I was quite stuffed after I finished. It was reasonably warm day too (about 27deg C) as it is just coming on Summer over here so that could of had a bearing on it as well. The other week I did a 150km ride, partly over the same route as this weekend's ride, but with more hills (steeper and longer too) and wasn't as tired when I finished.