Summer, well done 
That must be much the same length of walk that I did many years ago in the Chepstow/Tintern area. I was with a friend who walked faster than I was used to. On the way back I was suffering from the worst cramp I have ever had so the last few miles were rather unpleasant. Your avearge speed works out at 2.75 mph which very good for such a long walk. I reckon the first half of our walk was something like 4.5 mph but the last part was a crawl.
My typical walking speed is about 3.5 mph on shortish walks but that's on pavements rather than in the countryside.