I just learned unicycling 3.5 weeks ago and on Wednesday I got a KH26. Like you I have a burning leg, my right though. I also try to keep my weight as much as possible in the saddle and when it does, it does actually ride much more comfy. I can easily do the 100 yards and free-mounting (a little less easily), but yeah it is hard work. I train every evening and refuse to use my car to climb on top of the uni. With all I’ve learned this month, I am already looking forward to the end of the next. Lots and lots to learn.
I am experiencing the exact same problem, though at around 1500m now. As said elsewhere, a mantra helps. I also found setting targets, but keeping them “moveable” makes a difference. For example on my best rides along the promenade I’ll set a target of reaching that bench, then when I get there my next target is that post box, the petrol station, the next bench, etc. Having these distant but visible goals also helps with keeping my head up and looking forward which has a knock-on effect on balance too - the wobbles seem smaller in the context of a 100m stretch than if you are focused on a point 2-3m in front of your wheel.
I was nowhere near 100 yards at 9hrs practice, so well done!
Skipped over all previous posts and will offer what I’m sure has already been stated:
Legs burning when a new rider? That’s pretty normal. Your body is still learning how much power needs to be applied to keep things under control. And you’re not sitting down. Make sure the seat is at a proper height, keep reminding yourself to sit down, and try to relax. Then just keep riding, taking breaks when you burn out. Your quadriceps will get stronger than they probably ever were, and your body will learn how to ride efficiently.
Enjoy!