My boots were pushing my lower legs too far forward and in an "upright" position I was still flexing/straining my quads.  I was in constant pain and couldn't ski for more than 20-30 seconds without stopping.  I swore that if it didn't improve this was the last time I'd ski.

Finally, on the last bit of the last run, I unclipped my top boot strap and though they were loose I was able to stand up straight.  No more fatigue.  That's when the fun started...