A long distance relationship would need marriage prep MORE than a standard, face to face all the time, relationship. It’s easier to hide things or gloss them over, because there is limited time spent together.
This is not just some inconvenience the Church is imposing upon you. It is a necessary exercise in getting you talking about major issues that all married couples have to discuss. If you are going to spend your lives together, then six months to a year to prepare is not too much to ask.
If it is impossible for the couple to attend marriage prep together, then it is time for that couple to reevaluate their commitment. Either now is not the right time, or these two people are not meant to marry, or it is time for them to get serious and relocate one of them so that they CAN do marriage prep.
Parishes are willing to work with you. My DH and I were going to college in Chicago and planning our wedding back home in California at the same time. (With much help from moms and stepmoms!) We did our preCana and marriage prep with the priest at the parish we attended in Chicago, and he sent all the materials back to our home parish in California. We did do a little redundant prep at home, but not much… just to put the priest at ease that we had really done it… and then we got married only a few weeks after graduation!
Do not look at this as a roadblock. Look at it as the Church being loving and responsible toward you. They do not want to bind anyone in a lifelong, unbreakable, sacramental marriage without making sure they understand what they are really doing. Because wouldn’t it be wrong to obligate people to those requirements without fully educating them first? A year spent in prep can prevent 5 spent in counseling or divorce court.