I've been married for 26.5 years
You first meet your future husband and you're not sure if you even like this guy. You go out on a few dates and think, yup something could come of this relationship. You go out on more dates and suddenly realize you can't live without this man, he's your life. You think about him all the time, you hope he calls to set up more and more dates. You get married, spend all your time together for the first year or two. Then you have kids, life gets in the way and your mind isn't consumed just with him anymore. Sometimes you can go for 8 or 9 hours without thinking of him-your work gets busy, you kids need attention.... When you do have time to think about him and spend time with him, you appreciate him more and more, because you've been through the good, the bad, the ugly together.
I've been running for 25.5 years
You first are introduced to running and you're not sure if you even like to run. You go out on a few runs and think, yup something could come of this relationship. You go out running more and more and realize you don't want to live without running-it part of your life. You think about running all the time, you hope you can set a time for more and more running. You commit to marathon training-it consumes you, you do this for the first year or two. Then something happens and you find work is busier, your friends call for the movies, you don't centre your life around running anymore, you fit your running around life. When you do have time to think about running and spend time running you appreciate running more and more, because it's carried you through the good, the bad, the ugly.
Kristen, I think you're still in the dating/engaged stage....

(just my .02 cents)