Cycled 63k in the blistering heat, and very slowly. I narrowed the ‘problem’ down to 3 things – a problem with my bike, a horrible headwind, or my legs just being knackered. After oiling everything and seeing there being no problem with the bike, I suspect a conbination of the latter two.
On finding the campsite in St Quentin I met an Aussie couple, Jo and Alex, going in vaguely the same direction as me. Alex confirmed there was a horrible headwind, so that solved that mystery! We decided to cycle together tomorrow to Laon.