I have run lots of half and full marathons. I would to start with get your shoes checked have you run under 300 miles in them and are they the correct shoe? Go to a professional store and get a gait analysis they will check the way you run and what shoe you are in. Many problems stem from this and it could be a cheaper start in the long term than paying for physio. Good luck
I think that it makes sense to focus on the weight loss. I would see a professional about your knee and get expert advice. The knee might even be a symptom of some other injury/ deficit. Like a back or foot problem.
The volume of training would depend on what your goals are.
To finish, just spending the time on your feet will help.
I would be monitoring the ?injury. See the doctor, see what triggers it, etc.
Good nutrition during a half helps. Use the aid stations. Stay hydrated.
Pace yourself. Don't start out too fast. Don't fall into the trap at running somebody else's pace.
Up a mountain? Add in lots of hills in your training.
There are lots of beginner, intermediate and advanced running programs available on the net. Some are free.
Be conscious of your safety. Not a good idea to run alone in bush, mountain or isolated areas.
Sometimes the event you plan to race has some decent race information. Let you know what to expect. Some even have training programs specifically for their event.
If you're watching your nutrition and exercising the weight should come off anyway. Do whatever works for you and is the most motivating. Setting yourself realistic goals can also be helpful.
Running is as easy or as complicated as you make it.
I should also mention that the marathon is up a mountain...