Is it a sin to have premarital if you plan on marrying the girl?

So I waited for a long time, i'm in my mid 20's and so had my girlfriend. We both will attend long graduate school programs (3-5 years) so any marriage plans are out the window. Well we decided to stop waiting because we would marry each other if we could so our hearts are in it, but of course we cant. Interestingly, we dont feel guilty because we love each other, and when we do finish school I'd marry her. So now what?