I agree that getting a Half Marathon in will really improve your focus on the training. I can't see the 10k race contributing much to the aim in this case. The priority will be building the endurance base needed for the marathon, and running 10k in training will soon be considered a short run (I hope!). The 10k race during marathon training tends to be used more for sharpening up speed.
While you are gradually increasing the running mileage, you can continue to improve your endurance through walking which is low impact. When I started, I often ran a few miles, and then went out for a long walk of 3 or 4 hours. Its not the sort of fatigue you get in the marathon, but its a start.
October may seem like a while away yet, but in reality you have less than 3 months available for training. I would settle on a plan, and then commit to it.
Where are you running the marathon?