People change over time. It does mean that you will put the relationship first, though.
I think being with someone for decades requires an unusual amount of dedication above anything else.
I think marriage (once all the lovey dovey first flush is over) is about compromise. You compromise everything from the start and the measure of your commitment is the ease with which those compromises are wrought. As to the decades, well one is pretty much like another after a while. I have a sneaking suspicion a lot of marriages hold for a long time out of habit and friendship rather than anything else, which is no criticism. If it works, it works.