ahwiles » stuff that works for us:
spend more time and money fixing/buying her kit than yours - you may be able to 'nurse' an out of tune rear derailleur for a few more rides, don't assume she can do this.
remember to feed her regularly - a little something every 20mins or she'll get cranky.
she'll start off colder than you, and warm up faster, and cool down quicker, etc. be patient as a saint when she stops to put on / take off layers, be even more patient when you end up carrying her layers.
pay for some coaching, Campbell coaching were good for us.
some days, if it's cold, windy, raining, sleeting, she'll not be up for it. this is fine.
front-wheel lifts are the most useful skill ever, and not as easy to learn as you may think.
15k is a long way.
reds are difficult.
10k swoopy blues are your friend
There speaketh a man with a happy relationship...
OP, as a mountain biking girl who only started due to my boyf here's how I see it...
Make sure she has a good bike that she likes and make sure it works. When she's already out of her comfort zone a crappy gear change or ineffective brakes won't fill her with confidence or enthusiasm. My first proper bike was a Specialized Camber, after a couple of years riding old hard tails and struggling with them it was a revelation and one of the best things I could have done. As my riding progressed I realised that it was actually too big for me so I sold it for a Giant Trance, I never got on with the Trance or it's sketchy gear change and totally lost interest. Hell if she want's a bike painted in pink glitter then do it no matter how stupid you feel being seen with her!
Same goes for equipment - I found having the right trainer and pedal combo made a huge difference. As does a good jacket/helmet/gloves.
Loose the attitude. This thread title makes it sound like you couldn't care less, she'll soon pick that up and you'll cause yourself no end of grief.
When you go out with her don't grudge the time spent on tow paths etc. All time spent on a bike is a good thing, being with her doing it is a bonus, if it's not then why bother at all? Equally she needs to understand that you need to ride without her to do the stuff you want and she should be happy to let you.
Find a good local skills coach, maybe even a female specific trainer/group, and encourage her to go. She will probably struggle to take advice from you - I can't take instruction from my boyf, nor him me - so leave it to a professional.
Don't over face her, that doesnt mean never go near a hill but be mindfull of her fitness and ability as it grows. Just because you can roll the steep chute with the rocky berm at the end doesn't mean she can. Start small and build it up, theres nothing more motivating than feeling like your improving.
Personally I'm a fairly outdoorsy girl, I like the rain and mud (we have horses) and going fast. My bf knows he can bugger off down the hill at speed and I'll make my own way down, he also knows I hate him being condecending or treating me like a child. Find out how your OH wants you to treat her - should you stop when she is far behind? Does she want coached through the tough bits? Do you need to help her strip the bike to put it in the car?
It's not rocket science, honest
I've gone from my only bike experience being a road bike in my early teens (20 years ago) to last week picking up a Pitch Pro in preparation for a week in the Alps later this year (my first ever proper holiday!) and I can't wait. I'm not the fastest or the bravest on a bike but I love it, my bf has been brilliant, plus he loves the fact my interest in bikes means that bikes in the lounge, parts in bits all over the kitchen table, Follow Me in the dvd player and an eBay watch list full of Lyriks, shifters and full face helmets is the norm.
It goes both ways though - I have been into horses for years and bf has immersed himself in it too, he has a horse, rides when he can and takes a huge interest in the horses, my riding/training and all the kit involved. I only got on a bike because I felt guilty about the amount of time he spent doing my 'stuff'... If only I'd known then!