Battlestar Galactica: The Mini-Series
It's a one season, one episode three hours long. For some reason it shows a 2nd episode that's not on Prime, I believe that isn't true. The next is a series Battlestar Galactica ('04)
>first episode I saw was about Admiral Adama retiring and his ship being put out of service
I think that is the mini-series 3 hours (2003) and then a year later it was picked up as a series (2004)
After seeing an article on the best shows on Amazon Prime, I noticed BSG there.
Start with the miniseries, then move on to season 1.
This is the miniseries. It served as a pilot for the TV show. I don't remember if it was widely known at the time that they were trying to do a whole TV show, but it was relatively well-received.
Next comes the series. There are four seasons overall. There are two made-for-TV movies - Razor, and The Plan. Razor can be watched after the season 2 finale. The Plan can be watched once you have finished the whole series.