Hi Emma, well done on getting a much-coveted place in the VLM next year. Don't worry. You have plenty of time. First advice, forget what everyone else around you seems to be doing . It's your body and your experience and you are unique.
Second thing, set yourself a realistic target. If you are a new runner, then this is probably going to be to get round the course (+ without injury - pointless if you trash yourself.)
Breathing will come just naturally if you don't overdo it. You may find that you are trying your hardest on every run. That's not how it works. Maximum effort is for races, and short ones at that! As others have said, build it up nice and slowly. You will learn the pattern of breathing that is right for the effort you are making. Your brain will do this for you, it doesn't need to be consciously controlled.
Think of spending time on your feet rather than running miles. It doesn' t matter if this time is walking or running in any combination. Just try and push it out for a bit longer over time. So spend 20 mins a session walk/ running, then in two weeks, increase it to 30 weeks, and so on. Does that make sense? Let us know how you are getting on.