Your doing the right thing, take her on increasing ride lengths and "slowly" add more speed as she get's used to riding. My wife of 20 years had never had any interest in bikes till one day, she got tired of the kids telling her how great it was to ride with dad, LOL.
I just added distance and speed slowly over the next riding season and now she's been a "hardcore" co-rider for 13 years!! She's so good at it now, I gave to reach back and touch her leg to make sure she's still there and unlike other co-riders, that smack the stuff out of their riders when it gets quick time, she leans with me and think going fast is fun..
If you do stupid stuff (wheelies/stoppies), ect. right off the bat, she won't be around long!! Remember, she's trusting "her life" in your hands so don't get over your head trying to stay up with the "single riders"! I've ridden hundreds of thousands of miles since 1976, many alone but the best have been when the Mrs. is working the backseat!!
BJ Ondo, "Old Fart"
1980 GS1100E Suzuki
"The 1980's Hyabusa"