Unfortunately, there aren’t really any particularly attractive waterfront paths in Manaus. But there are a few decent sections.

Waterfront:

Av. Marquês da Silveira. Roads on each side of a small canal. 5 km out and back. ROUTE MAP

Av. Lourenço da Silva Braga. This is the main road along the Rio Negro in Manaus. It’s a busy road choked with cars, but there is a decent sidewalk. Water views along about half of this road.  ROUTE MAP

Parque Rio Negro. About 0.7 km of pleasant paths in this riverfront park. And there’s a sidewalk on the R. Cinco Setemnbro bridge, which leads there from central Manaus.

Beaches:

As for beaches, there aren’t any that are particularly long.

Praia da Ponta Negra is 0.7 km long, and is runnable. There’s also a dedicated path along the beach.

Praia da Lua (Moon Beach) is the most scenic and longest beach (1 km) in the Manaus area, but it is not particularly accessible.