There are absolutely hiking groups. They're are even hiking groups geared towards single people. I have friends that met SOs through them. Also, I think something like a book club would be most likely to be organized through another organization first (a large workplace, a Facebook group or Reddit sub geared towards a city/ county) that would start as a discussion forum and move on to in-person get-togethers.

And if you don't come across any of those maybe you live in a small community and need to look within the next larger community. And don't be afraid to organize it yourself. People like all those things (hiking, gardening, books). Also, re: nature and gardening, you can usually also volunteer with local nature preserves (our country has a local conservatory). Good luck out there!