So I used the command line of GPSBabel to interpolate points at 10s intervals as described in the documentation:
gpsbabel -i gpx -f track.gpx -x interpolate,time=10 -o gpx -F newtrack.gpx
Unfortunately that generated an extra spur of travelling but that was easily deleted with Adze, and as it was only a single point it didn't disturb the total time significantly.
I can't set my 12XL to record at 10s intervals though as on a day hike the memory would be exceeded and it would also drain the battery; at 2 minute sampling it lasts a very long day, unlike my Forerunner 110 which only manages 3 hours. Perhaps due to its high sampling rate?