HTFU! And plan on some longer training rides next year.
You want to go metric even before you hit the metric century age?
I'll normally plan on a few extra miles when I ride my age or a metric, just to make sure I hit the mark.
Was the map off by 420 miles in a 15 mile stretch, or did rock faces/buildings/tree cover make you lose GPS lock and add spurious excursions to the track?