I applaud you doing so many miles on a Trek Navigator. I have a Navigator 2.0 for cruising the neighborhood. I have tried longer rides, but would definitely need a different saddle and DH along if to put it on a bike rack!! I consider the Navigator more of a comfort bike rather than a hybrid (my hybrid is a Trek FX), but for longer rides, one of the road bikes would be my choice. I think if you want a different bike, look at lots of styles, including performance hybrids (flat bar road bike) and road bikes. Buy the one that makes you want to smile and ride, and comes from a great LBS that you click with.

Enjoy the rode,
P2