I would say it's aggressive as a new runner, but six months is definitely a good amount of time to train; however, there are many things that should factor into your decision...
- Are you good about keeping to a training schedule without missing days? If you are one that will likely miss runs due to various issues (vacation, sickness, family obligations, lack of motivation, life in general, etc), then I would take your running ramp-up a little slower. Maybe train for a 5k for 10 weeks or so, then run a 5k, then onto a 10k training plan with a goal race, then a half marathon training plan and race, and finally a marathon training plan and race. This will likely put you more on schedule for Goofy in 2018, but it gives you more time to see if you like running and how your body responds to longer distances. It will also greatly lower your probability for injury during training.
- Your age and health. If you are younger and generally fit, I would be less worried about injury and the ramp-up. How is your overall health? You may want to check with your doctor and see what (s)he says about your planned running ramp-up. Although you are a beginning runner, have you been doing other aerobic activities that put you in good or at least decent shape? For example, if you've been cycling or swimming (or some other aerobic fitness activity) for quite a while and already have a good aerobic capacity, then Goofy 2017 is probably fine. However, if you are just beginning to run in order to get into shape and/or lose weight and running will be your first aerobic fitness exercise (or first in a long time), you may want to take it slower.
- How does your body react to increased physical stress? Historically, has your body performed well when adding physical stress? Give yourself an honest assessment of this. All bodies are different, so if you have reacted well to increased physical stress, then again, Goofy 2017 is probably a good goal; otherwise, there is no harm in taking it a little slower and going for Goofy 2018.
Whatever your decision, I would suggest finding a good training program. Most of your runs should be at an easy pace (meaning roughly 60-90 seconds per mile slower than your goal marathon pace). A lot of beginning runners tend to overdo training initially (i.e. run too fast and/or too far), so please find a training plan with a nice easy ramp-up and stick to the prescribed paces. I would say that most folks here use either Galloway, Higdon, or Hansons plans, so those are probably safe places to start.