I think a more complete way to say it is:

-- Both intervals _and_ slow distance improve "cardiovascular".

-- Intervals improve shared _central_ cardiovascular (capacity of heart,
lung, major arteries + veins). Slow distance improves specific-muscle
_peripheral_ cardiovascular (density of capillaries)

-- Intervals also produce important "peripheral" muscle-specific
improvements: lactate threshold and lactate tolerance.

Greg Fangel wrote
I believe that interval training improves your VO2max
and LSD improves your endurance, cardiovascular delevery system.

Results from a long-term health+exercise study were reported recently in
several newspapers. The key finding I remember was that the higher the
_intensity_ of exercise sessions, the greater benefit to heart health and
length of life. Intensity was found to be more important than Duration for
long-term health. I think these results were for non-athletes.

I think that it depends on what type of skier you are.
If you are a skier at the American Birkie that slugs it
out for 4-6 hours, I would believe that LSD training
would work better for you as the bulk of your training.

-- My experience is that for the goal of "making it thru the distance" in a
running marathon, just one long slow distance workout every two weeks is
enough -- if I progressively increase the time by say 10-15% in each
slow-distance workout. Just have to get started on that program a
sufficient number of months before the race.

-- If still have time for a second workout each week, seems to me it makes
more sense to use it something that will raise the lactate threshold.
Because the higher the threshold, the faster you can go in the race without
getting anywhere near that threshold. Especially for a hilly course, where
it's kinda hard not to push up close to LT.

So seems to me one intensity workout plus one slow-distance per week is much
more valuable than two slow-distance workouts per week -- even for a 4-6
hour finisher.

Ken