I agree with Zeke — you shouldn't pick the marathon as a first race. The risk of injury is very high. However, it can be done if you aren't worried about time and don't mind walking a large percentage of the course. I was in your shoes and actually ran a marathon after not running for about 10 years. I completed it but I was in pain afterwards. I didn't run another marathon until 2 years later — after I trained properly and while nothing beats the feeling of accomplishment after completing your first marathon, I was in considerably less pain after the second one. The smart thing to do is to train at least a year and then tackle the marathon, but it can be done if you're lucky enough not to get hurt in the process. Looking back on what I did, I know I was more lucky than smart in doing the marathon first with only a short period to train.