Yes, you can COMPLETE the Detroit marathon in the approximately 18 weeks that you have to begin training for the 26.2 ~ How fast you run to complete the marathon depends to a large extent on natural ability ~ both physical and mental ~ you may have to walk a part of the 26.2, but hey, so what since it is your first. In order to avoid training injury, just go at your training in a conservative manner with the idea in mind that you may have to walk part of the marathon course. Running a marathon as your first race after 10 years, is not the best way to begin running again, but it has been done, and you can do it. There is a lot of beginner training information available if you search it out. You have set a goal that is a good thing not a terrible thing. Have fun.