I don't have any practical advice for you since I was not raised LDS and missions are a new concept to me.
One thing I have jokingly heard is girls who date guys that go on mission seem to married the next guy they date after that. Though I am sure if it is meant to be you two will figure it out. Just keep in mind what 2 years really is! I am thinking back to who I was 2 years ago and I barely know that girl who had just came back from 2 years in Ireland. Still immature compared to myself now but less so when I left home 3 years before that. In two years I have moved three times, gotten married, got pregnant, left my husband who was put in jail and then had a baby!
Your going to change so much in two years and probably more because of the mission. What if you change and she doesn't? She must find her own life and self while you are not there to hold her up. Perhaps if she is strong in her faith she could do a service mission? When you return you will be full of life and hope and have a million wonderful mission stories. If she does not have as much to show for her two years she will come to resent your growth and wish she had awesome and interesting things to tell to. Encourage her during your mission so she can return the favor. If all she does is pine for you then she might make the mistake of holding an image of you in her mind that is better than you can live up to as a normal mortal man and at the same time she will have done nothing to discover herself and what she is meant to do in her life.