There are a few different options here. If you wanted to use this warm up race as an "all out effort" to gauge your fitness, we'd recommend doing this no sooner than 5 weeks before your goal marathon to allow proper recovery time and a full taper.  There will be key runs at 4 and 3 weeks out that ideally you won't want to miss.
With regards to your training, simply replace the long run of that week with the half marathon race, and don't worry about the extra miles - the max effort of the race will fatigue you enough. You might also want to make the few days leading into the half marathon a little easier by adding an extra rest day or dropping the intensity or mileage slightly, acting as a mini taper so you are fresher.
Alternatively you could use the warm up race as an opportunity to practice your goal marathon pace. Again, just replace the long run in the plan with your race but keep the rest of the week the same.  Don't stress too much about adding extra mileage before or after your race, the consistency throughout the rest of the plan will see you through.