I don't know much about navicular so I shouldn't have an opinion on it:). But from the research my vet told me about it is that navicular is caused and worsened, by NOT enough conditioning. 1) I'm not trying to talk you into it or out of it, really, just discussing. 2) I would think a strong, physically fit horse would stay strong and physically fit longer then one not conditioned. 3) I would never say just suddenly start taking him out for hours at a time every day. I'd just say a common sense build up. And for this sport I don't call the conditioning I beleive you need to be "heavy". Build slowly to 3-4 times a week for 3 hour rides. Then about 2 weeks before a competion increase to 4-5 hours. 4mph, is not slow, but it's also not that fast. It does require some long trotting. I'd say the most important thing to look at in your situation is the terrain you'd be riding over. Rocky, hard, smooth, soft, hills? Etc.
Becky
It would depend I think on his x-rays.... Much of the pain in NS is inflamation of the deep flexor tendon from moving over a changed, rough Navicular bone. The more the tendon moves over that rough surface, the more inflamed it gets. exercise improves blood flow and improves the condition, too much causes damage. It shows up in hard work, disappears with rest.
I'd just have to try doing what I'd like to do with him, and really read him as to whether he is holding up and enjoying the activity. Right now, he gets really jazzed about going for a ride and he steps lighter and faster than when he is clunking around the pasture all bored.