This may be Earthblood by Keith Laumer and Rosel George Brown
- Humans are almost extinct, most that remain are mutated or
This is the major premise of the book. The hero is an almost-unique pure strain Terran
- Earth's location is forgotten, but main characters find it.
In the latter part of the book, the hero and his comrades' search for Terra is successful.
- Earth surrounded by blockade of derelict ships.
The alien Niss fleet orbits Terra but is abandoned/filled with dead Niss.
"I see our dreadnoughts, all in station, orbiting the enemy homeworld.
But no one answers our signals...."
-- the last surviving Niss
- Aliens lack the ambition/survival drive of humans - a broken leg might be a fatal injury.
Sounds familiar, but a quick check did not turn this up
- Galactic civilization is decaying.
Again, a major premise of the book.