Sounds wonderful Ben. If it's a popular cycling route then it's probably not mtb singletrack of the gnarly kind. That kind of stuff (rocks, roots, logs, steep climbs...) I can manage about 15 miles max. before I'm whacked.

BTW, if you're riding mostly smooth gravel and asphalt, you might think about swapping out those knobby tires for some with lower rolling resistance. Will save you a ton of energy, esp. over the distances you're talking about. Check out Chain Reaction or Wiggle for great deals, or your local bike shop. Let me know if you need some advice, and I can recommend a few models.

Enjoy the bike!

Svend