There are 2 blocks of extended data. Each one tells you what it is:
<extendedData dataType="distance" intervalType="time" intervalUnit="s" intervalValue="10">
and
<extendedData dataType="speed" intervalType="time" intervalUnit="s" intervalValue="10">
So, the first says each value represents a distance with the interval between the values is time in seconds of size 10. That means that each number is the distance (in meters) taken at 10s intervals.
The second blob of data are speed readings, again at 10s intervals. There used to only be one ExtendedData block and I don't actually see the point of the second one as the speed can be calculated from the distance over time data in the first block. I only use the first block of data for the graphs you see here on Slowgeek.