Jogging in Venice

The sea side promenade starting at the Doge's Palace (Palazzo Ducale) is your best bet. It's very wide, very long, you won't get lost, and the scenery is spectacular:

google map of promenade

The nice thing is there is no auto traffic and it's quiet first thing in the morning - perfect for a jog. The promenade is a good choice. When we were there they had an organized run and that was part of the course but you can really just jog randomly without any problem.

That's probably the longest, straight stretch you can find. You could also go across the Accademia bridge and run around the Salute and the along the fondamenta on the other side back towards the ferry port.