Surely the important thing is to have some common interests, and not be identical in every way. My partner skis pretty well, bikes a bit lamely. I ski quite a lot with her – but go away on other trips too. Ride with her only occasionally. And I don’t go to her book group and never knit. Kids and other stuff we share. Somethings I deliberately avoid taking up to give her some space.
So the truth may be a little bit less clearcut that TJ claims, after all – his wife doesn’t post on here often does she? So even he appears to have an unshared hobby 😉
In this case surely the question is how attractive MTBing is to a girl, either to do as a dating activity, or as something which suggests the OP is fit and worth a shag? And you won’t get those answers here.