In the Swarzędz area 10 km east of Poznań, there’s some nice running along Lake Swarzędzkie (Jezioro Swarzędzkie). There are paths on either side of the lake, but not a full loop around it.
East Side: The more developed path is on the eastern side of the lake. From the parking area at the Swarzędz sports complex (running track, indoor pool) at the southeastern end of the lake, run north on the paved path for nearly 2 km to Cieszkowskiego Rd. The path is mainly wooded, and there are some nice views of the lake. Bonus: lit at night!
West Side. There’s also the Darniowa path that follows the western side of the lake for 2 km. This wide, wooded path is more rugged than the east side and a bit away from the water in parts. The southern end is off Strzałkowska. It’s possible to connect to it from the southern end of the east side path, using Strzelecka Rd. It’s not that scenic, but there is a sidewalk. The connection is 2 km,.
Lake Swarzędzkie is also the site of a weekly parkrun.