Thanks for all the great comments. I suspected that more practice was going to be the key and that seems to be the case here. I am definitely looking at ways of improving my tracking.
The one thing I tend to look at in panned images is the amount of background blur and in general, I like it when the background has so much movement in it that those elements are unrecognizable. I realize that to get this level of motion blur, the shutter speed relative to the subject has to be slow. This increases the risk of not getting the shot. As an aside, the amount of blur I show in the posted image is roughly what I would like it to be.
As for the focal length, distance to subject, constant speed of the subject. I had considered all of those things when I took the shot. I used a moderately wide (45mm focal length on a FF camera) and left myself plenty of space. I did want to position the vehicle towards the left hand side of the frame to give myself a bit more visual space for the motorcycle to move into. The image I posted is a significant crop from the original and I suspect I could give myself even more leeway; this is the original image.
Bill - I like your idea of using a merry-go-round to practice on. Unfortunately at this time of year those are hard to find in this part of the world... I suspect I am going co-opt my wife or daughter into driving around the block a number of times for me to practice...