My DW splits the driving chores with me just about 50-50. We're running a 43' tag axle Bus pulling a full size Chevy pickup. Two to 2 1/2 hours in the seat at a stretch and the other snoozing on the couch if not needed for complex directions or other duties. Start driving on more remote routes where you feel comfortable and expand to things like Interstates, mountains and then the big cities. DW has run Houston, TX and Atlanta, so I have no concerns, but I do sit in the copilot seat to help with lane changes when the traffic gets crazy, as she does for me when I'm in the seat. Comfort will come and will depend on your total time doing it rather than reading books. When your partner is riding copilot have him explain things one at a time to you so you can practice it right then. Things like climbing hills, running snake like mountain roads and even pulling into a fuel stop are easier learned when you do it with help. My only problem with DW's driving the rig now is getting her out of the seat so I can have my turn. We've run over 51,000 miles in the last 30 months so seat time is easy to come by.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad