Your Fjord is looking fantastic. I think it would help you out a lot to ride with some endurance riders and see how it goes. It sounds like he may have the mental drive that is necessary to do endurance.
What some people don't realize when they start out is how fast you actually have to ride in order to finish in time. There is not much walking done in an endurance ride, you are trotting and cantering most of the time for a few hours. This is where some of the heavier muscled and draft type breeds lose their motivation and it can become too difficult to push them on even if they are physically able to complete.
Pay special attention to cooling measures for your horse, he will need more help in this area than the lighter breeds.
It may be helpful to time your horse and see how fast he goes trotting for a mile or two. He needs to finish a mile in at least 10 minutes in order to do a 25 mile ride in the 6 hours allowed. When you do the math, that seems to allow for tons of extra time, but the 6 hrs. Includes the vet check, and you would be surprised at how quickly your time gets eaten up by narrow spots, steep downhills, bad footing, water breaks, etc.
The reason this is helpful to know is that some people who have tried to use "non-endurance" breeds have not been able to get their horse to sustain a pace and have had to give up for that reason. If your horse easily does several miles in a half hour, then you know there is a higher chance that you will be able to do endurance.