Yes, i am sure you can. I hadnt run a marathon when i did my first IM, although i had been doing triathlons for about 7 years so i had built up endurance. I would maybe try an olympic or half first. You are very young, there's no rush.
I would say 20 miles a week is fine for now, the marathon isnt until April right? If you start working really hard now, you may burn out or get injured. The 20 miles a week will give you a good base and then you can start to follow the plan when the time kicks in.
I am not a big fan of sports drink/gels so i often drink apple juice on runs and eat sweets. From what i can tell re fruit juice, it's just sugary drinks anyway. It doesnt have the electrolytes but I dont tend to need them.
Also I would work on speed. I ran a 4h6 marathon last year and i was probably in 51 min 10k shape then. Everyone is different but i would say you need to be going much faster to stand a chance of 4hours.
I would be really mad if this was me. If they had set everyone off on time, you would have made the cut off. These races aren't cheap, that's before you think about all those hours training etc. I would complain to them.