Also, you most likely cannot stick to your diet because you are restricting your intake too much or your body is craving all kimds of nutrients because of a combination of things such as your activity level being high while spending a prolonged period of time in a caloric deficit. When you say about 18 being a standard runne BMI that is not the case. Okay, it is to find elite (are you elite?) distance athletes with a BMI over 23 but having a low BMI does by any way mean you are at an optimal BMI for running. There are lots of fast runners out there with BMIs of 16/17 who clearly are not healthy (and clealy aren’t going to be able to keep this up for a long time without permanent consequences) who would actually be running a lot faster if they added some weight and got their BMIs into the 18/20 range but they are too scared of the calories to do this. If your BMI is too low then you are not going to have the strength or the power to reach the next level in your training, optimal strength-to-weight ratio paired with great mileage is what makes champions, not obsessing over calories in the orange juice they had for lunch. Some people can be healthy at a slightly underweight BMI but those people are rare, in order to be optimal/healthy at those sort of weights for long periods requires a genetic predisposition meaning you have to be born with a super light frame, good muscle tone and a raging metabolism. Think: Kenyan people. They are very fast and when given the opportunity, a lot of the, eat what they want and as much as they want. Their bodies are programmes to burn and perform. Yours might not optimise calories that well and to be honest, restricting is just going to slow your metabolism long term.