Your love for Ranger comes through in your post. You cared for him so much, and you made the compassionate decision in letting him go. I know that's very little comfort to you right now, but you've taken on his pain, the most selfless thing you could do for anyone.
How beautiful that you had some time together at the park and talked before you parted ways. It reminds me of the last two walks I had with my Dante. The vet said he didn't show any signs of heart failure (yet) but my gut told me that I didn't have much time left, so I savored every little step on our evening walks. Those last two walks, he would be too tired to walk back, so I carried him (he was a 7lb chi), and he would rest his little head on my shoulder and close his eyes, one time we just sat on my porch, with him in my lap, and watched the sun go down. He'd always run straight back indoors for a treat, but he was very calm, he was looking around, as if taking it all in one last time. He knew, too.
Its been nearly two months for me, I lost mine very suddenly on May 4th, and typing this, I still well up, but everyone here sharing their grief and their love makes me feel less alone, so we're here for you.