There is no right answer for this question. You alone will be able to make the decision. There are different training plans available online, most of them for free, preparing you giving you information about how you should plan your runs and how long before your race you need to start.
From what you posted you are running about 25 miles per week. In my opinion, that mileage is enough to prepare for a half marathon since this is your first. The week before the half marathon, you should be able to run atleast a 10 miler. Don't do this 2-3 days before the race as you might not be able to recover in time for the race. Try to do it the previous weekend from the race. First race will always be nervous but there are lot of people who do run/walk combination. Some do more run and only some walk. Some do more walk and some run. There is nothing wrong with taking a walk break if you need one.