Just my two cents:
Working trot is the most efficient gait for endurance, conserving energy over the course of 25 miles and allowing your horse to regulate itself while still travelling at a decent speed. This is about 8-9mph. An extended trot (my boy can cruise at 13-14mph) puts too much strain on the ligaments, and it would be better for the horse to lope. However, you generally don't want to do this over the majority of the ride, and loping puts more strain on the ligaments than does a steady working trot. This is the reason the AERC changed the rules for finishing an LD - too many riders were loping the 25 and their horses were suffering because of it. Now, you place in the order that you pulse down. For example, say you come in first, but your horse isn't pulsed down. Someone comes in afterward and pulses down first - they take first place and you take second (or third or whatever depending on how many riders pulse down before you).
In addition, you need to take into account the horse's metabolics. Loping too much can mess with these, and they don't always show up in a vet check. You need to know your horse, and by the time you notice something is off, it may be too late.
Do an experiment: Take your horse out and lope him for a while. Usually, he's going to start sweating and really get wet a lot faster than he would if he were trotting. This is evidence of two things: your horse is working much harder and is therefore sweating more; your horse is losing water much faster.
Also, horses sweat differently from humans. In humans, only water and salt comes out in our sweat and our blood concentration changes, signaling us that we're thirsty and need to drink. Horses, on the other hand, sweat isotonically, meaning whatever is in the blood comes out in the sweat and their blood concentration doesn't change, so it doesn't signal to the horses to drink more and replace not only the water, but also all the electrolytes and nutrients you're losing. If you don't, you're going to be in trouble and the normal vet check may or may not pick it up.
One final thought: As other riders have said, there are a lot of variables. I know that some riders do lope their horses, but usually they have been doing this kind of things for decades and so know their horses and signs of trouble inside and out. Then again, I also watched a veteran rider almost loose a horse this year at a ride because she was overridden - and it wouldn't have been the first horse he'd lost. Get someone in your area who has done this for a long time to ride with you and mentor you. If you only began doing this in the last couple of years, your horse likely is not ready to lope a 25 and you could end up with a serious injury, most likely to the ligaments as they take at least a year, sometimes longer, to strengthen. This is not a sport to take lightly any decision about how to run your race - 25 miles or more will turn a small bad decision into a very big problem. And if you plan on ever doing 50's or longer, you need to teach your horse to pace itself on a 25 or it will not be able to mentally prepare itself for longer rides.