OP doesn't say where he's located, but here in the PNW you're allowed to ride on most of the highways once you're outside of the city centers. Heck we've even got an ultra that uses the interstate for about 250 miles of the course. The Cannonball! is a 275 mile race from Seattle to Spokane which gets on I-90 (3 lane, 70mph limit) outside of Issaquah and exits the highway at the near edge of Spokane. Quite a few of the Seattle Randonneurs' long courses use I-90, SR-2, SR-20, SR-9, and many other 55mph divided highways for their routes.
It can be a bit freaky at first, but just like any other road riding around traffic, you get used to it.