"maximising every session without overtraining?"
Mel, the answer is that every session should have a goal. That doesn't mean every session should be hard. Some sessions the goal is improved flexibility, general aerobic endurance, anaerobic endurance and capacity, neurological training, or simply creating blood flow and warming the muscles up for active recovery.
If every session has a goal, you will likely maximize the session by accomplishing the goal. If you train without goals then it's hard to maximize, since we don't know what we are trying to maximize.
"training topics: im very slow but can endure - never ever focused on speed. Where do I even start to focus on speed in 3 events? while weight training, doing yoga, working on core (and not losing endurance or getting injured!) where to start?? Thx! :)"
KS, thanks for the question. Don't fall into the trap that speed is only going hard. Speed is simply moving quickly. This can be for a few seconds, or longer. If you really want to improve your ability to fire muscles, and increase run cadence, bike cadence, etc, then you need to work on shorter intervals. Try surges in your run of 7 to 20 secs, at fast speed.
Remember that speed is simply the product of Force, (f) and Velocity, (v) in a movement. The only way to run faster is to apply more force(f) into the ground with each step, or take your steps faster(v), or both. In biking, we can apply more force to the pedal, or spin at a faster cadence, again f and v.
You've got the f down with the strength work, but work on the v! Work on keeping technique good when increasing the velocity of the movement.