How to sum ab all kilometres of running from one year?

Isn’t that already in your stats? Or do you mean a rolling year instead of the calendar year?

The yearly total distance (current year) is in the stats as ​@Jan_Mantau said. If you want an export to play with, you can do that via this feature (but it exports literally everything so it may be an overkill):

https://support.strava.com/hc/en-us/articles/216918437-Exporting-your-Data-and-Bulk-Export
 

You can also feed your data e.g. to Runalyze (website only, not an app) for more detailed and user-friendly analysis.