Songwriter/Performer: Dan Fogelberg
While Dan Fogelberg recorded songs that were more rock-based through the 70s, he says this song came to him one night while he was on vacation in Hawaii, enjoying a night in the hammock, looking up at the stars. He says "it just seems this song was drifting around the universe, saw me, and decided I'd give it a good home." (from Wesley Hyatt's 1999 book "The Billboard Book of #1 Adult Contemporary Hits (Billboard Publications)").
You can find the song on Fogelberg's "Phoenix" album, released in 1979 or on his 1982 greatest hits album. The original recording features acoustic guitar (played by Fogelberg himself) and flugelhorn.
Other artists have covered it, and I've included some of them below to give you a few more options, including Floyd Cramer (pianist), Babyface, Brazilian pop group Roupa Nova (recorded in Portuguese with the title Bem Maior), and Irish/Australian singer Damien Leith.
Longer than there've been fishes in the ocean
Higher than any bird ever flew
Longer than there've been stars up in the heavens
I've been in love with you.
Stronger than any mountain cathedral
Truer than any tree ever grew
Deeper than any forest primeval
I am in love with you.
I'll bring fires in the winters
You'll send showers in the springs
We'll fly through the falls and summers
With love on our wings.
Through the years as the fire starts to mellow
Burning lines in the book of our lives
Though the binding cracks and the pages start to yellow
I'll be in love with you.