Should I have a physical before I run a marathon?

Of course. Make sure you are visiting your doctor before running even the half marathon. Moreover, if you are 40 or above, you should have an exercise echo stress test to prevent CVD.
Yes, definitely. Best to have a baseline health assessment and thorough examination/blood work, etc., prior to training for a marathon, let alone running one.
Absolutely. Even if you are in good shape, its best to see your primary care physician or sports medicine family physician to get a full physical and green light to run a marathon.
Yes. Definitely be checked by a doctor before training or running a marathon.
Absolutely. Always good to get medical clearance. You will be stressing your body and you want to be sure you are ready for it. Also provides baseline to compare to if there are concerns down the road. Stay well hydrated ! have fun.