Yes doing exercises can help but,,,,even a body builder who trains everyday will have "burning thighs" after skiing 30-40 IN AN INCORRECT STANCE !
90% of skiers ski with weight on their heels and as speed increases they tense up and lean back even more !
Just watch beginners and,,,,,,,look at their ski tips often off the snow,,flapping !!!! A sure sign of heel pushing!
Try this,,to solve the thigh issue! ( assuming you are fit

!
Begin with mind. Think "pressure at all times at ski tip"
Think relax and let skis do work.
Think soft with edges
Think,,,"too fast" then go across and up slope ! Let slope slow you down!
Then;
Put pressure on big toe at all times
Do NOt go so fast down hill
Enjoy your skiing,,why go so fast,,,,
Here is the critical hint ( which will seem wrong to you

!
The faster you go,,,,the softer you must be with edges
The faster you go,,,,the less body motion is required.
The faster you go,,,the less muscle you need to tun skis
The faster you go,,,the more forward you must be! ( to be ahead of the skis in the turns)
The faster you go,,,tiny "adjustments" are multiplied 10 times !
Just move your nose a few millimeters to the right as you soar,,,,,see what happens!
Hope this helps you,,this season!
Pavel