It’s hard to describe or even list all the trails in Croatia and we will definitely not try to do that.
Instead, we want to give you some links where to go searching for the trails. Or if you need direct advice, contact us and we’ll try to help you.
Regarding the running-oriented areas, Zagreb county, Istria, Kvarner region (especially around Rijeka) and Dalmatia (around Split) have decent running groups. If you are looking for some company on one of the islands, it’s a bit harder to find it. If you’re going to Dubrovnik, think more about some other activities and not running. 🙂