A minor bug.
I currently have units set to Imperial. If I go to calculations and select "How are these values calculated", then under Marathon shape there is "Target kilometer per week" - it says 51. If I simply change my distance units preference to Metric, then it still says "Target kilometer per week", but says 81.
I guess that, when I have units set to Imperial, it should say "Target miles per week" instead. This is confirmed when I go to the "Activity per week" graph, where the target distance per week is clearly marked as 51 miles.
P.S. I am using the official, hosted version.