If you're upping your mileage ahead of a goal marathon or half this year. you have to adjust your calorie intake accordingly. Here is a rough guide:

There are various running calorie calculators available online and most sports watches track it too. As a general rule, the average runner will burn about 100 calories per mile (1,6km).  You have to consuming enough calories to account for that, even on your rest days.

To get an idea of your daily calorie output, try using the Schofield Equation, to estimate your Basal Metabolic Rate (BMR). This measures the rate of calories your body needs to function at rest. As with most of these types of equations, this isn't a hard and fast rule, but should act as something of a guideline.

Women 18-29 years: 14.8 x weight in kg + 487 = BMR
Men 18-29 years: 15.1 x weight in kg + 692 = BMR
Women 30-59 years: 8.3 x weight in kg + 846 = BMR
Men 30-58 years: 11.5 x weight in kg + 873 = BMR

Once you've calculated your BMR, you can then take this figure and apply it to your level of training (or 'activity'):

Inactive men and women: 1.4 x BMR
Moderately active women: 1.6 x BMR
Moderately active men: 1.7 x BMR
Very active women: 1.8 x BMR
Very active men: 1.9 x BMR

In the early stages of a marathon programme – if you're training for about an hour, four times a week – you will most likely to fall into the 'moderately active' category (assuming you have a desk job and not a highly active occupation). 

So, as an example, a woman with a BMR of 1336, who is 'moderately active': 

1336 x 1.6 = 2138 calories required.

As the training load increases and you are doing longer and more intense sessions, you would also be classified as 'very active' and this figure would then change accordingly.