Thanks @greenroofer. I guess my expectation was that we would be something like your experience at a speed approaching 8-10mph when we were moving and long stops every 5 miles or so. They happily do the 4mile school run without a break at those sort of speeds.
In reality we were struggling to sustain 8mph. Some of that was poor gear selection but it was the stops that really killed us. Looking at strava we rarely did a mile without stopping for some reason or other.
When they want to they can apply themselves. Heading North on the bridge with the wind behind us was okay but they wanted to stop and look at things. Coming back into an really awful headwind and with another rider just in front of us, they both just got their heads down and powered over without stopping doing 8-10mph on the way up the bridge and 12-14mph down the other side. It was probably our fastest mile of the whole trip.
It may be that their heads weren't totally in it today (end of holidays and me only announcing the training ride this morning when they were already engrossed in their screens probably didn'thelp). It certainly is harder having 2 of them to divide my attention (with consequential jealousy tantrums) or the route be broken up by sections on public roads which then required a certain amount of safety instruction and higher stress levels.
I will look at next weekend's weather forecast and maybe do the Airdrie -Bathgate route to give us some flatter and more consistent offroad cycling.
We'll definitely be looking for an early start time to give us the biggest time window, so we can take a relaxed approach.