What differences will I see if I run 3 miles a day 7 days a week for four months?
Not an expert as such, but my opinion is that every person is different in how their bodies react to training of any kind. It would be difficult to determine what differences you would feel as this is dependent on what type of off training/dietary plan you were following, your muscle density, your metabolism whether fast or slow and how often and what you were eating/drinking during your training plan.
That said, if this is your first time out I would err on the side of caution(don’t you just hate it when your doctor tells you that) and consult your doctor in the first instance. I’ll take it that you are already of some fitness if you are capable of running 3 miles a day 7 days a week for 4 months (any rest days, these are much needed to help the muscles repair).
If you are pelting the 3 miles your cardiovascular will greatly improve along with stronger thigh and calf muscles(especially if you are following an aerobic/weights workout).
Train smart, train safe, run well and rest, rest, rest.