Update. Replacement hip still going fine after 5 years of fell running. Still capable of 5-6 hours on typical Lake District terrain and run 2-3 times a week on fells with remaining days on road. Distances from 3 mile aggressive burn-out to 25 miles steady endurance. Only difference the hip replacement has made is no longer indulge in manic descents. Partially due to not rolling/bouncing so well (68 now) and a 'back of the mind gremlin' telling me to preserve the joint.