You have to tell them the truth. You say you're afraid of hurting them but what do you think is going to be worse, being dumped after a year or being dumped after 10 years? You're wasting your life and theirs.
If I had a gf who dumped me after 5 years and said that she stopped loving me and wanting to be with me after the first year, I would want to do things to her that would require me to go off to a large, state run complex for 5-15 years of my life.
Sure maybe 5/10 years is a bit out there, but I absolutely know of couples who have been together that long who I know shouldn't be together and it's only a matter of time (or they're delusional beyond everything..... or have kids). Even if it's being together 3 months longer than you want to be, that's still 3 months both of you could be doing something better with your lives. You don't have the right to waste the other persons life like that.