If you want to run a sub 3 marathon then you need to focus on marathon training. Slowly increase your mileage and the long stuff will get easier. 4:04 off of the little training is actually pretty good.
Don't worry about mile time and all that. Focus on the marathon. You can run some shorter stuff during training just to assess but don't focus on it, especially the mile. The occasional 10K or half would be far better.