If you go to the Boathouse Cafe in Essendon you can do a circuit around the Maribrynong River (just under 1 hour). You could also do a walk from Fed Square and head towards South Yarra and walk to Chapel/Church Street along the Main Yarra Trail (35 mins one-way, just under 1.5 hours return to the city). There are coffee shops at either end of the walk.