It depends mostly on your body weight, not on a fixed distance number. Using the ACSM running equation, a 70kg (154lb) runner finishing in 4:00:00 burns about 3,250 kcal over 42.195km, and the same runner finishing in 3:00:00 burns about 3,175 kcal: weight moves the total far more than pace does. Enter your own weight, distance and time above for a figure calibrated to you rather than a generic average.
The ACSM (American College of Sports Medicine) metabolic equation for running: VO2 (ml/kg/min) = 0.2 x speed(m/min) + 3.5, converted to energy using the standard approximation of 5 kcal burned per litre of oxygen consumed. It is the same equation behind the Marathon Ballot Treadmill Pace Calculator, published in ACSM's Guidelines for Exercise Testing and Prescription.
The ACSM equation estimates the average runner well, but any single formula is a group estimate, not a lab test. Individual running economy varies with stride mechanics, terrain, wind and fitness, and this calculator also assumes a flat course, so it does not account for hills. Treat the total as a solid ballpark, not a lab-measured figure.
Only a little. The oxygen cost of covering a kilometre by running is close to constant regardless of speed, so total burn is dominated by body weight and distance. Pace has a small effect because the ACSM equation includes a fixed baseline metabolic cost per minute, so a slower finish time means slightly more total calories for the same weight and distance, not fewer.
Gross. The total includes the energy you would have burned anyway just being alive for those hours, which is standard practice for exercise calorie calculators and matches how most running watches report the number.
This calculator converts your total into average USDA FoodData Central figures: one slice of cheese pizza at 285 kcal, one 12oz regular beer at 153 kcal, one medium banana at 105 kcal, and one standard energy gel at 100 kcal (the GU Energy Gel spec, in line with most other brands). These are averages for typical servings, not a nutrition plan.