thanks James – yes, I tend to go out and push myself when I do ride – the road ride was nearly 70% in what Strava calls ‘Threshold’, the mtb ride less so (about 40%) because there was a lot of singletrack.
Riding more often and/or for longer is probably what’s needed.
I tend to start getting problems with fatigue and cramping (despite lots of electrolytes) on the odd occasions I do go over 4 hours on a single ride.
Looking at doing the big Dog again this year, maybe what I need is a plan rather than my usual ‘up the ride time a bit in the month before it’ approach.