Definitely consult with your vet & farrier - they're the experts. That being said, I had a beautiful mare that came down with "pre-navicular" inflammation a couple years ago. She had extremely low heels on her big ol' Thoroughbred looking feet, in fact her heels were just about crushed & she was walking on the bulbs. She had a little bit of navicular bone degeneration but not too bad. With the help of my awesome farrier she was riding sound in about 3 weeks & within a year she had grown heel. She did need special shoes (the Natural Balance shoes were her saviour) & shoeing every 5 weeks, but it was worth it. I did condition her for LD but never competed her as she had back issues that caused her some soreness after about 20 miles. I sold her & bought a new horse (my 'distance' horse' haha) that came down with laminitis shortly after I got her. (Guess I've not had good luck with feet - btw, both mares have wonderful hard black feet - go figure!) Just the same, both my vet & farrier (& some kind ridecampers) say that the laminitis (which was a much bigger deal than the navicular) should not prevent her from doing distance. So far, she's had no evidence of any more foot pain, our first event is planned for Feb 2 & I really think her conditioning will be a much bigger issue than her feet! Good luck!
Bany in Fla