Hello, Vili. This is a great question. There are no hard and fast rules to fasting, such as how many days it should go on or even what to fast (it doesn’t necessarily have to be food; it can be anything that takes up a good deal of our time in life), and sometimes fasting will not result in answers to specific questions, especially if for some reason God wants us to wait for an answer or wants to stretch us to lead us to deeper levels of trust in Him. The primary reason for fasting is to get our focus more on God rather than have questions answered, though there are times during a fast that answers may come.
None of us here, as far as I know, are doctors, and no one here is your physician who would know your state of health, so it is outside of our limits to be able tell you whether or not you could do a 10km run while fasting a meal for a day. Therefore, I would recommend asking your doctor .
It seems you were expecting some more definite answers, so I hope this helps you somewhat. To my knowledge, there are no biblical mandates or instruction as to specific numbers of days of fasting for different issues. That is between you and God.
Praying that as you fast, your relationship with the Lord will grow deeper in ways that you’ve never imagined!