Back in the late seventies I began collecting a sci-fi series. There were just a ton of them, and they were all pretty standard pulp sci-fi hero stuff, all about the same hero and, I believe, by the same author... someone who later became famous and wrote these under a pseudonym. My mom chucked the books while I was overseas, and I can't remember what they were. I thought it was the Lucky Star series by Asimov, but those were only six books.
I suggested this in the comments as wild guess as the description was "pulp fiction, long-running from the 1970's" which put in my mind the Perry Rhodan series. Turned out to be correct as mentioned by the person asking, even though this series started in the 1960's.
As confirmed by the OP in comments, this was the Perry Rhodan series.
The first 126 novels (plus five novels of the spinoff series Atlan) were translated into English and published by Ace Books between 1969 and 1978
The story line starts in 1971 with the first manned moon landing by U.S. Space Force Major Perry Rhodan and his crew, who discover a marooned extraterrestrial space ship from the (fictional) planet Arkon in the (real) M13 cluster. Appropriating the Arkonide technology, they proceed to unify Terra and carve out a place for humanity in the galaxy and the cosmos. (The concepts for two of the technical accomplishments that enable them to do so—positronic brains and starship drives for near-instantaneous hyperspatial translation—are direct adoptions from Isaac Asimov's science fiction universe.)
As the series progresses, major characters, including the title character, are granted relative immortality. It is relative in the sense that they are immune to age and disease, but could suffer a violent death. The story continues over the course of millennia, including flashbacks thousands and even millions of years into the past, and the scope widens to encompass other galaxies, extremely remote parts of space, parallel universes and weirder cosmic structures, time travel, paranormal powers, weird/cute/aggressive aliens and bodyless entities (some with sheer god-like powers).
The first few English translations appeared with these covers: