Hi Mike,
I hope to go from 0 to marathon in less than a year, I doubt that I'll be first across the line even with a 2h head start. Yes it is possible but your knees will probably decide, you should speak to a specialist.
For a beginner I think reading is just as important as running. I was also sportive when I was young so I was surprised to learn that running isn't just about putting one foot in front of the other. Eg breathing, inflate the stomach to suck air out of your lungs, great technique whilst walking during the intervals. Lots of contradicting information, find what you feel is right, you can always change as your experience grows.
Intervals are great, you're doing 2:6 splits (run 2 mins, walk 6 mins) and you can build on that. Then you can do one session intervals and another long run which could be 20 minutes at first. As you improve you can add a hill (small slope). Your intervals can become longer 5:2 and/or faster 1:1. Read about v2o max and Intervals.
The clever part (which I didn't do) is to build up slowly, you'll make faster progress in the long term. As you said your not out to break records so when you do your LR it's better to run 20mins slowly than 15mins too quickly, this will become more important when you're running further. Also 'calm' weeks, 3 weeks progress, 1 week cool.
Being a beginner, if you are ill or injured then form goes down quickly. It will go up again, but it's hard for us. On the flip side, loads of PB's just waiting to be set and begging to be broken. Be careful you could catch the running bug ! Good idea for the races, gives you objectives. You don't have to do marathon training from day 1, there are loads of training plans on the net, for 5k then 10k and HM.
You won't necessarily lose weight by running for many reasons but you'll feel 100 times better. There will be fat people passing me on the marathon because they have trained harder than me but they are still overweight even after the hundreds of training miles. As you run more, you eat more and you tend to have more treats. If you've lost 5st, congratulations, then you'll know more than most about nutrition. Water is very important, read about hydration and dehydration, if you dehydrate even a little which you may not even notice it will affect your performance.
I've got a Garmin 410 on promo but 110 is just as good. Great for knowing pace and/or distance which is encouraging. I like the Internet site, the records and the map.
Core training is important, core muscles support you when running so you don't become sloppy. Hips forward, back straight. Swimming is good cross training.
I've heard that building up too quicky, ie marathon within a year, then your body can suffer, perhaps not tomorrow but in years to come. A reason to build up slowly.
The next person to comment could say entirely different to me. It's trail and error, experience is everything. Perhaps join a running club.
Good luck and enjoy