Wanting to print first poster with D800 image

A few weeks ago I took my new D800 to a swim meet. It was my first outing and I was just fine-tuning the settings. I took a photo of a swimmer doing butterfly. The best shot was when he was at the further end of the pool. I was using my 70-200 but he was still pretty far away. His mom saw the image last night and wants to print it as a 20x30 poster to hang up at his graduation party. When I crop it down to get him mostly edge-to-edge the dpi is approx 128. What are the steps that I can use to make this image the best quality poster that I can?