If you are mainly training for endurance, you should switch to emphasizing speed in your training. You will see results in a month or two.
Basically, to improve, you need to get faster, not be able to go for a longer period of time. You can already paddle at race pace for 100 seconds or so. If you have been racing, you know you can go the distance. The difference is who can go the distance fastest.
But I am tired at the end of the race, so I need to train for endurance. Everyone is tired at the end of the race. The world champion is tired at the end of the race. Something is wrong if you aren't tired at the end of the race. Being tired at the end of the race is normal.
And yes, technique needs to be emphasized. But going slow does not improve your technique usually, unless you are first learning a new technique. You need to be able to do the technique at high speed. And you will find that if you are fast and strong, you will be able to do moves that you couldn't do before. Your technique will improve.
How do you get faster? Mainly by training fast. Your muscles have to be used to going fast, so that is their normal pace. If you train slow, your muscles will be used to that pace and will go that pace in the race. If you have long rests between runs in practice, you will be improving your speed more since you will be able to go faster in practice with complete rest than if you start again when you are still tired.
That is why I do not normally recommend the "lactic tolerance" type workouts where you do a run, then have a short rest, say 10 seconds and repeat. For one thing, with this type of workout, you aren't going all-out, so you don't increase your speed. Secondly, you don't have time to evaluate your prior run, get feedback or get psyched for the next run. The quality of the runs goes down.
I believe it is important to have high-quality runs in practice. If you have high-quality runs in practice, you will do the same in a race. If you have low quality runs in practice, it is hard to change that for the race. So, do what you need to in order to have high quality, focused runs in practice. Wait until you are fresh and ready. Don't go if the poles are swinging. And quit before the quality of the runs goes down.
Speed should be trained early in workout, not by doing sprints at the end of a workout. I suggest doing short courses of 20-30 seconds for time and penalties. And some workouts where you do shorter courses of 10-20 seconds for time and penalties. And some workouts without gates for each of these time periods.
And speed should be trained at least once or twice per week. Some people can do ok with just one speed training session per week, but most people need 2 and some people should be doing 3 per week. In any case, doing zero speed training per week is a mistake and you will not get faster with such a program. Practice paddling fast and it will become a habit and your times will improve.
Here is a link to a mixed martial arts article about speed training:
http://www.athletesacceleration.com/speed-training-combat-athletes/
