This sounds spot on. At 25, I find myself thinking these thoughts. Someday I'll finish this project. Someday I'll take this trip. Someday I'll spend a full week at my grandparents.
And I'm realizing, as you aptly said, that the mere act of telling yourself "someday" is enough to comfort you that it will happen, that you shouldn't worry about it, until it's too late.
My grandfather fought in WW2 in the French resistance, leaving his family at the age of 17 to live in the wilderness for 3 years, stealthily blowing up Nazi trains and doing various things like that. He got a fairly prestigious military decoration for it at the end of the war. He never really talked about it when we were kids though - he probably wanted to keep us blissfully ignorant.
For about 2 years, my cousin and I talked of taking a few days to sit down, ask him questions about it, get him to share his story - and film the whole thing and produce a short oral history documentary out of it. We kept putting it off because I live in the US, she lives in Switzerland, my grandpa lives in France, and occasions were rare.
Then he died last February. Despite a few surgeries, he was in fairly good health at 88 (still drove around, gardened, fixed stuff around the house, etc.). On a Monday morning he felt unwell, on the Wednesday morning he was gone.
I'm trying to minimize those "somedays" now so they don't hit me in the face 100x when I'm 45. And yet it's always so easy to just say "someday".