La Plata was founded in 1880 and was masterly planned to become the regional capital of the Buenos Aires province, with the first stones set on November 19th, 1888. Today, La Plata is a city of 1.2 million people (the 3rd largest in Argentina), with ~800,000 inside the "circunvalacion" (the "casco urbano") and ~150,000 outside (the "con-urbano"). Around the city is the circunvalacion, a hub of recreational activity with football pitches, football-tennis courts, space for picnics and mates, exercise equipment, and small restaurants dotted throughout. You'll find wonderful street art & murals. The University Train ("Tren Universitario") wraps around the north-to-east of La Plata, with extensions toward Los Hornos currently under construction.
There is not an obvious starting point, given the route could be started/completed at any point along the city's outer ring. I chose the monument to La Plata found at Calle 13 & 32 given the theme & where it sits at the city's entrance toward the more populated suburbs between Tolosa and Buenos Aires. The route can then be completed clockwise or counter-clockwise. There is also a snaking walking & bike path along roughly ~30% of the circunvalacion, so as a demonstration, I tried to stay within the inner side of the circunvalacion.
Heading counter-clockwise, you'll head past numerous monuments and landmarks, including the Estadio Unico Diego Maradona (~1km, calles 32 y 21-25), a Skate Park often with a good amount of action (~2km, entre Bv. 82 y Calle 28), the entrance to Los Hornos neighborhood (Calle 131 y 60), the old train station headquarters and City of Cats ("Ciudad de Gatos"; ~10km, Calle 17 y 72), the artsy Circunvalacion Station (~11km, Calle 13 y 72), and the UNLP Medical & Journalism college buildings (~14km, Bv. 120). The rest of the circuit along Ave. 120 is pretty transit-heavy*, so I took the route into "El Bosque" from km 14-16 where you'll pass the only water station, Gymnasia Stadium, the Museum of Natural History, and the Estudiantes Stadium. From here, you'll continue along Calle 1 passed the current train station (Calle 1, Diag. 80) before meeting back up with the circunvalacion at Calle 32 before a gradual downhill jog back to Calle 13.
*I've interchanged both routes and will always end up almost exactly 20km, so thinking is safer and prettier to go through El Bosque.