Couch (and massively overweight) to half marathon

My suggestions would be to make sure you can walk 5km (or even 6km) comfortably before you start your Couch to 5km. So you might need to add an extra week or 2 at the beginning, depending on what distances you are walking at the start. The Couch to 5km will push your speed, so you need to have distance and kind of ‘muscle fitness’ down, or you won’t be able to keep up or will get an injury.

Secondly - i would think about doing each week of Couch to 5km 3 times, not just twice. That way you are getting properly fit for each stage before you move on, and you are letting your muscles develop and increase each step, thereby meaning less injuries. Don’t underestimate how your knees and feet are going to feel with all this work! Give them a chance to ease into it, and rebuild themselves.

There are 2 ways to repeat a week (or do it three times) - that is to simply do it as written twice, OR do Monday session twice, then the next day session twice etc etc so you do a slower build up.

you may worry this will leave you less time at the end to increase your km, but in my experience, it is far easier to go from running 6km to running 7km, than it is to go from running 1km to 2km. Give yourself more time at the start getting the foundations and the basics right, and the rest will take care of itself as your fitness improves.

I think it's absolutely possible, but if you don't radically change your eating habits from the very start, no amount of exercise is going to combat a poor diet. You need to focus on your nutrition from the very beginning.