How important is it??

I finished my first HIM this year in my goal of sub-6, and I did exactly zero intense zone 4 and 5 stuff, outside of races. 

give as hard as you can...... make it hurt!! Intervals, and high intensity speedwork increase your body's ability to process oxygen! Therefore you can go longer at "any" pace!

Simplified 2 cents.

With my limited knowledge I will advise as follows. 

If you just want to finish, no need to do speed work.  I get what you mean by  you call yourself a zone 2 kinda girl.  I love zone 2.   

Not sure what you mean by finishing strong.  If you mean you don't want to fall across the finish line and have to be carried to the medical tent then all you have to do is train the distance+some extra in each sport and race smart and take your time.

I did next to no speed work in my training for my half and my IM, I zoned 2 if my work out was a 30 minute run or a 9 hour bike ride.  I finished what I concidered strong, meaning my body was in good shape, I was smiling and I could go for a hike the day after both my half and my IM. That being said I did my half in 7 hours and my IM in 14 1/2 hours.

Now I am figuring out how to go fast, I think it will include training out of zone 2.

Good luck!

Reasons you should do speedwork no matter distance you are racing:

1. You are teaching your body to work at a higher capacity aerobically, then you would have otherwise.
2. Doing fast sets teaches your body efficiency. Doing the same pace over and over will just get you the same pace over and over. Efficiency = free speed.
3. When you are racing long distance events, you are using mostly slow twitch muscle fibers. Once your ST muscles fatigue, they look the Fast Twitch muscle fibers to carry the load - if these muscle fibers are untrained, then you will slow down considerably.
4. Go fast, its FUN!

To add of what Mike R said:

Increasing your metabolic fitness which is usually optimized at your threshold pace (more specifically @ Maximum Lactate Steady State) is essential for endurance athletes as it allows them to carry over a greater power (or pace) over time. It also correlates very well with performance, usually the greater the power/pace at threshold the better the potential to perform over time (duration) of course with proper training.

Some of the training adaptations achieved training at tempo (usually know as Z3), Threshold (usually refer to Z4) and Vo2 max (usually refer to Z5): Increase in muscle mitochondria enzymes, lactate threshold, muscle glycogen, inter-conversion of fast twitch muscle fibers IIb to IIa, increased plasma volume, muscle capillarization, increase stroke/maximal cardiac output, etc.

That been said; there is a delicate balance between how much intensity you should shoot for each sport and when you should do it. This has to be planned based on your main event of the season and you sport weaknesses. Also while swim and bike adding these intensities can be less stressful on the body for running the build up should be much slower given the wear and tear the body is exposed to. Doing too much too soon of intensities sessions could be a quick recipe for injury.  You also have to realize that you can’t work all at once; it is more efficient and easier to handle the total load by planning different cycles with different focus. i.e. work 6 weeks on increasing threshold, next 6 weeks on VO2 max, etc.

Finally the time main even will dictate when to add what sort of intensity training. An athlete training for an IM would benefit from this early in the plan as the main race get closer and the training switches to specific training Z4 or Z5 training won’t be as important as Z2-Z3. OTOH, someone training for an Oly could work on endurance (Z2-Z3) and VO2 max early on the season and as the main race gets closer the focus should switch to Z4 specific training.

That's actually the area where you should be doing the MOST of it.

Why?  Seriously, I'm not trying to be argumentative I'd like to know.

1. Its easier on your body than the other two sports.

2. Anything you don't like to do is probably going to help you since it's probably a weakness. Its easy to do the things we are good at, but Champions train thier weaknesses.

True.  But since the swim is so much shorter than the other disciplines, I've always heard that you should spend the majority of your time trying to improve the run and bike.  That's assuming that you've reached a level of proficiency at swimming, where your technique is serviceable and you're, let's say,  MOP.  Obviously, this would not hold true for elites or highly competitive age groupers.

From my experience with running, the amt of speedwork seems to vary a bit depending on your physical makeup, but in general, nonelite runners seem to improve a lot more by increasing lower-intensity volume (significantly) versus increasing speedwork intensity and/or frequency over a nominal level such as once every 1-2 weeks.

If you're doing NO speedwork, sure you will likely benefit from it, and quite rapidly, in fact. However, those gains (VO2/lactate type gains) are known to taper off very quickly - weeks, in fact. Lots of "off the couch" athletes misinterpret their initial rapid gains in VO2 to be real aerobic gains, and get frustrated when after 6-12 weeks, they can't seem to improve at all on the same low-volume program.

Still, the best path to definite long-term improvement for endurance events like 5k+ races and triathlons is volume. (For nonelites.) If you run or bike literally double the volume of your next competitor who is of similar age and genetic endowment, there will be literally no type of interval training they can do to overcome the gap.

My 5k PR from age 18-30, while running 30mpw on average (peak 40mpw): 20:30 regardless of what type of hardcore intervals I did. (Even up to 3x/wk.) Avg running pace was 7:15-7:45/mi

 My 5k from age 30+ while running 60+mpw on average: 18:40. Maximum of one speedworkout per week. Average pace 8:30-9:00min/mile, which is a LOT slower than my prior running pace. 

 Take race results from even longer races such as half marathons and 15ks, and the differences are so huge you can't even compare. (Like 30 minutes on the HM.)

 My take from the running perspective: One speedwork session every 1-2 weeks is definitely enough, even for AG wins and top 5% finishes. Much more important is consistent volume. 

Also it is incorrect to state that VO2 or threshold gains are known to 'taper off' quickly. If by that you mean that you stop making gains at that intensity in a short period of time, that is incorrect. They will level off only if the load remains the same or until you reach your max potential and for that it will take a lot of training. Anyway, to avoid any of this usually training plans follow periodization.

Some of the benefits of intense training that Jorge mentioned carry over from one sport to another.  E.g., an intense session swimming can (among other things) increase stroke volume, and this helps running and biking.  So, if you can get some of those benefits with a lower impact and lower risk of injury, seems like a good thing to me.

Oh, and it isn't only champions who train their weaknesses.  Some of us mediocre athletes try to do that too...  

To Mike's point, if you are swimming you might as well be doing some harder efforts; there is little point in your typical triathlete just putting in lots of zone 2 swimming.  The reason is that very few triathletes will ever approach the weekly volume required to benefit from a ton of zone 2 swimming so they might as well do some harder work for the limited time in the pool.  IMO, the majority of time a triathlete spends in the pool should be near race effort (except wu, cd and drills).

Further, as far as a level of proficiency at swimming, I don't think any triathlete should stop working on the swim.  Since most 12yo girls that do club (or even just summer) swimming would hammer almost all triathletes, putting your swim into maintainance is a mistake for most (unless your swim is far and away your strongest sport).

Shane

Agree 100%  And I fixed that last part. ('me" refers to me, not you!)

I wasn't offended.  12 y.o. girls kick my @ss all the time in the pool.  Now I understand better.  Thank you for the explanation.