I've done a lot of research and gone to Dengies website for advice and what he's getting is about right. I've got a weigh tape so keep an eye on this regularly. I took him off the pony nuts as many horse nutritionists recommend a higher fibre diet for better digestion. He just used to inhale the pony nuts whereas he takes his time over the HiFi and beet. He also gets two big nets of hay a day. I'm not doing 18km every day, I worked up to that over a few weeks. He does get ridden probably 4 or 5 times a week and one of those rides will be a longer one.
To do shorter endurance rides up to 30k or even more native good doer types can often do very well on a fibre only diet. Looking at your chap he looks the picture of health and well being on it. I think you are absolutely on the right track OP.
The problem with non long fibre feeds for these types is not only do they have too much
sugar and starch but also get eaten or inhaled as you say so quickly leaving the horse at risk of ulcers or colic from gorging and then periods of nothing to eat as they would have used up their sustainable amount of calories. Also from a nutritional point of view the recommended amounts to feed to get the vitamins and minerals in the right quantities would be far far too much for a native good doer.
My Haflinger does endurance rides of 20km and hacks of the same distance in the cairngorm foothills on a diet of hay, oat straw and fast fibre with pink powder and fit/min supplement. I would suspect your pony will be much the same.
If and when you are competing in the Golden Horseshoe on Exmoor or the world champs in Abu dhabi then it would be a different matter of course, but that's for next year,....:-)))