I would try and make sure you do some kind of consistent resistance or strength training too. It will help with injury prevention. Doesnt need to be too much or anything grand. A yoga class, pilates, body pump or this http://www.runnersworld.com/strength-training/10-essential-strength-exercises-for-runners
I would say you would be better doing a decent swim session in a 25m pool about 3x a week. Try and get up to distances of about 1500m, maybe even further. If you look up triathlon swim sessions, you should be able to find them. You should do some drills as well as general swimming. Going and trashing out 300m in a lake/quarry everyday is not going to help you that much and its risky.
MrTim I trained for comrades last year so i was doing some long training runs and i tended just to buy stuff whilst i was out. I wasnt scientific at all, my thought process was 'what do i fancy' - buy it, eat it.
I take gels during marathons and half marathons but i hate them. I prefer sweets whilst out training.
I would do some of the shorter runs at a faster pace but keep the weekend long run at an easy pace. Maybe make the Thursday run a tempo run, maybe 8m45 pace to start with or 9mm if you find 8m45 too fast. To get your target of 2hours, you need to run at 9mm pace so your body need to get used to running at that pace. I would also vary the distance at the weekend but i usually train up to 10 miles for a half marathon.
Re diet, i find that if i eat all my activity calories, i cant shift weight. I am not sure if i am just 'lucky' to be blessed with an efficient body but that's how it works for me. For example, if i ran 15 miles, myfitness pal woudl probably give me an extra 1500 cals or so but if i ate that, i would probably put on weight.