I remember when i first told my partner, i was going to do a half marathon. Her words were 'great, your going to be tired out all the time!!'. Over the past two years i have done about 11 half marathons, and am doing my first full marathon next year (hey! could have done one sooner, but i wanted to try a lot of different races first), which she is okay with. She actually realises i am more awake and alert, and got a buzz on, when i have done a good run, since i usually get tired quicker when i stop for ages, so her theory backfired.
I have kept it in balance though, i run three times a week, but do longer runs when i go out, and i limit my running talk. She has been to the odd event, and she is proud of me, but you cant be a dick about it, and think it wont impact on your relationship, if your letting it take over your life. I live and breath running, but i also plan my running schedule, sometimes weeks in advance, so i know its going to have the least impact on my home life.
People who are non-runners do struggle to understand it though, why we do what we do.