Working on somewhat the same thing here... The bug has already hit, somewhat. We haven't crossed the "new bike set up" deal yet but we're on the "ridable" path for starters by getting her old bike going. New tires, computer, and bottle rack are on it. Pedals next (threads don't work so it looks like an adapter is on order - got SPD shoes already too). I am not sure how to proceed with ramping up miles, but at least she has taken the initiative to get some things I've suggested and ride when we're not able to ride together.
I think a big part of it, as someone else pointed out, is to not go "hard" and let her have some input to where and the pace. We're not close (distance wise) so getting together isn't all the time. One ride we sorta rode from both ends to a closer spot for lunch (in the middle of the "shut down"). I brought a Wok, pot, and camp stove, brewed up some chicken penne pasta with simmered garlic, onion, and veggies. So she knows darn sure theres some "good stuff" that goes on with bike trips! As to the miles - that was a 65mi day for me on all new routes except for about 3-4 miles, 20 miles al new for her (round trip miles, so divide each by 2 for distance out for each).
I have a 30mi or so trip lined up. She's done over 20 before, so this would be her longest run. That is still in the "short and fast" category for me (under 2hrs ride), but I have an idea to do a point-to-point trip on an established trail or two (can't remember if it's 1 trail or 2 connected). Giving that 30mi run with making lunch on-the-go a full day would be pretty relaxing and what ever pace she wants to do/can do is fine - so long as we're not being chased by the weather.
I am not sure how else to approach the "how do I not screw up this deal" aspect. I'm not a relationship expert, I won't go in to details, but I do believe things happen for reasons. I am also not very competitive. So I don't think you should have to "try". In fact, I think the opposite - you shouldn't have to try hard at all. If you feel that way and always are looking over shoulders wondering what's going to happen or at what point the dynamics are going to shift negatively that doesn't sound like a relationship going in the right direction to begin with. I think everyone can find more peace in the world when they let go of how anything comes to pass (relationships included). I can say that, and I can understand it, and I do understand it, but I can also tell you that is damn near impossible. Of course, you can't just sit with a bag of cheesy puffs on your couch watching 80's show reruns for ever, so you have to do "something", but the "how do I not screw up this deal" is exactly what I'm getting at about letting go of how things come to pass. Who knows - she may look at what ever you throw her as a "challenge" and a "way to prove herself" to you. So the "game" may be all hers, not yours at all.
Best of luck.