Eat what suits you! Large doses of supplementary vitamins and minerals (such as iron) are not essential and produce no benefit if you are on a good mixed diet, but additional vitamin C in small doses is reasonable when fresh fruit and vegetables are in short supply.
Training helps you to sustain a high level of muscle glycogen if you eat a lot of carbohydrate. If you can, eat within two hours of your long runs and the marathon. This helps replace the muscle glycogen quickly and speeds recovery.
Do not change your normal diet drastically in the last week before the marathon, but decrease your intake of protein (meat) and increase your intake of carbohydrate (pasta, bread, potatoes, cereals, rice and sweet things), especially for the last three days when you should also be markedly reducing your training. This loads the muscle with glycogen. Unless you reduce your protein intake you will not eat enough carbohydrate. (Not all runners are helped by first depleting carbohydrate with a long run and low carbo diet and then loading – this can make your muscles very heavy).