burning thighs after about 30 to 40 mins of skiing

Agree with Wanderer, if you are trying to improve your skiing is excellent idea to have a few reference points. When I am having an off run I always try and revert back to.
1) did you pull downside ski backwards before committing to new turn.
2) Really accentuate the angle of the body to ensure you are keeping weight on downhill ski.
3) Body position, check uphill hand never moves behind your hip and line of sight is never more uphill than tip of downhill ski.

Thigh burn is normal part of skiing hard (am sure any downhill racer will concur). Yes you need to keep body in correct position but fitness is a massive part to enjoying skiing. If you can't enjoy a 30min run around in gym/park then cannot really expect to enjoy 4-5hrs a days on skis.

Ranchero has given great hints.
About those thighs and feelings after skiing.

We have a saying in my part of the world.
The Tuesday heart aches !

Why Tuesday and with heart aches ?

Here is typical scene.
A great ski week is unfolding. +40 cm. of fresh powder. 10 keen skiers who have paid a great deal of money to heli ski.

Monday is a constant pleasure of non-stop powder skiing.

Tuesday morning,,,,,well some "skiers" can not get up stairs, some have problems bending over to pick up trousers ( male and female ).
Some can not get into heli....
The Tuesday heart aches sets in.
No one waits for late comers. No friends in powder is the rule here.

In short learn to ski effectively and those burning thighs will not happen!

Apply those great hints and you will note how pain becomes pleasure,,,even after 5 days of skiing.

It is your choice.

Pa

For experienced skiers, fitness is not generally an issue. They may use a fraction of the calorie uptake of a person who is constantly fighting to keep their weight forwards. In addition, an experienced skier has learned how to carry out relaxed turns, where conditions allow, and even relaxed during a section of some turns.

Hence, there is no lactic acid build up. For top athletes, lactic endurance is part of their training. This is the same for skiers, although, it does not happen within a 6 day break.

During the gondola ride, there is complete recovery.

My Niederau ski day run record stood for 10 years, prior to being beaten by two 17 year old Austrian sh*ts. (Just kidding, I tell them that every year!) Later beaten by two super fit Dutch lads.
To get that record, I did not stop at all during the day. People made space for me in the queues, and I suspect the lifties were cranking up the gondola speed! I suffered nothing in the way of burning thighs. I was however, desperate for a pee!

I agree fitness isn't usually an issue for those of us who are experienced. However I still feel pre season training is benficial as it reduces the likely hood of fatigue, also chances of injury should you have an unplanned fall. Only last season a very experienced friend of my tore his Achilles' tendon within three hours of the start of the holiday. He never managed one squat before he ventured onto the snow!

Have to say I completely disagree with statements that suggest that fitness is not important for experienced skiers. Yes if you are doing cruisy skiing or straight lining it then can probably ski forever .

Piste day: Now think about someone skiing a steeper run with moderately aggressive high speed turns. Under these conditions you are heavily loading the outside ski, equivalent to leg a single leg squat of say 75% of your body weight. Now consider you want to ski 60 seconds like this you need to be able to continuously perform single leg squats for that duration and then repeat throughout a day. Am afraid that sort of skiing requires a decent level of both strength and anaerobic fitness.

Offpiste day: Now consider a more adventurous offpiste day with 3-4hrs skin before you start skiing, you need good degree of both leg strength, core muscles strength for your kick turns and high aerobic capacity. You get to the top and find 20cm of powder and medium steep slope which means you cannot just float over the top. This means you have to move 20cm of snow (depth) x 140cm of ski contact and 100cm (areas of disturbance or width) x snow density (we can say 80% air or 0.2g cm3) = 56000g or 56KG. Now each turn you perform you need to able to control body weight + 56kg and that is assuming weight is evenly spread between 2 skis (try squatting 56kg for for a minute straight). Yes new skis have greater buoyancy (that is why fat skis = fresh legs) and gravity helps you move the snow downhill but that is the simple physics of skiing, you cannot just float over the top on all terrain. Is also why the deeper the snow / heavier snow / breakable crust etc. means much more effort is required to ski it.

Of course experienced skiers are much more mechanically efficiency but to suggest fitness is not important is defying logic and physics. You don't even have to trust my calculation, check youtube and the weight lifting and cardio regimes of pros.

I totally agree ..as an older f**t I feel that you have to spend time in the gym doing squats step ups etc at least for 2 or 3 months before you go ..I am a proponant of the 30 minute (non stop 7 exercises done 4 times) workout which I do at least 5 times a week. I am looking forward to at least 3 weeks skiing in Jan/Feb so I want to hit the slopes at least half fit. Try Romanian Split squats for building up the leg muscles

There seems to be some confusion here, between fitness and strength issues. There is a third consideration ~ mobility.

For a track athlete, and I count over 20,000 miles in my athletics history, plus well over 250 weeks skiing, burning thighs occur as a result of lactic acid build up.

For a semi speed event, say the 800m, lactic acid will start to build up after 700m. For a 1500m runner, it is somewhat longer. But for a 5000m runner, LA buildup is much less of an issue.

And so with skiing. An average skier will come down the Marbachjoch in 15 ~ 20 minutes, whereas my best time is 2 min 42, and an average run is 4/5 minutes. I never have a LA issue. However, many of those returning the longer times will evidence tired limbs. The reason is that they tend not to have the required strength base in their muscles. Nor are they ski efficient. As a result, they are using up to 8/9 times more units of energy than a regular and experienced skier. Hence, I maintain that most long term efficient skiers do not have an issue with fitness.

Strength, is a really important factor. We all get into, errm, "situations".
Someone turns right across you, (yes, it is your fault, but you have to deal with it) You hit an unforeseen bump. Sheet ice. We have all done the bit of soft snow/fast piste, depending where the sun has been shining through the trees. So many different situations, but with one solution ~ we have to draw on our muscle strength to get back on course, and stay safe. So, in order to stay safe we do need strength. Strength is one of the two most important factors in skiing. We are talking predominantly about leg strength, particularly working on the quads.

Fitness and strength are the two most commonly confused issues in skiing. If you are in the gym, or on the road, running or cycling, your main gain is fitness, (your least need), but you will also gain in strength, so no real issue. If you are working on weights in the gym, you will gain in strength, (important), but not in fitness. Work on the quads.

The other, and important issue is mobility. It doesn't matter how experienced or inexperienced we are as skiers, we will all be hit with situations that require an extended leg stretch, a compressed and weighted knee. (Ouch, a sudden memory)

Confession time. I have suffered a couple of years of illness, something totally new to me! So I now know I have strength and mobility issues. Having walked Skiddaw earlier this year, I certainly know I have fitness problems.

A big winter approaches, Austria, France, Colorado, Scotland. So now, I am mostly working on strength and mobility! If it doesn't work, I will be doing a lot of painting!

Make sure you post those paintings on this forum for us all see, Dave. It wouldn't be the same without them.