Cramming that much video onto a single DVD is going to mean that it's going to look like crap.
13 episodes to one 4.7GB DVD = crap
26 episodes to one 4.7GB DVD = utter crap
13 episodes to one 8.5GB DL DVD = passable (but still a bit crappy)
26 episodes to one 8.5GB DL DVD = crap
Basically, you'd need a MPEG-2 encoding program to shrink down the video data and a DVD authoring program to cut-edit the video, add chapters and/or menus, and then burn the finished video DVD.
Another option which may give you better quality is to create a DivX DVD
but you'd need a compatible DVD player
to play it. Although I generally avoid promoting DivX, they do have a great deal of support by hardware makers of their 'format', making it the ideal way of getting more video on a single DVD without losing too much quality. I'm not sure if the software that can create these DivX DVDs have any editing features or if they do, how powerful they are. You may need to do any trimming/cutting/cropping with another program (VDub, most video editors, etc.)
DivX Ultra Certified hardware
DivX Ultra Certified software