I've just name changed for this thread and I am going to change straight back after I've posted.
I just wanted to say OP, I could have written your post. I'm 50, I've been with DH for nearly 30 years and happily married for 20. I used to absolutely love sex but over the years it has become less and less important to me, and now with three teens, a full on job, and the normal trials and tribulations of mid life coupled with perimenopause, I honestly just want to sink into bed quietly at the end of the day and peacefully read a good book. I am very happy, not depressed in any way, and have many things in life that fulfil me and bring me joy. But sex just isn't one of them anymore.
But it is very important to DH, and if it is important to him, then of course it is important to me. I loved your analogy of him going for a nice walk to make you happy, because honestly that is exactly how it feels to me. Not something I'd choose to do myself, but something I'm perfectly happy to do for the sake of my lovely DH. Often on threads like this, it feels like the only two states are either joyful, lustful sex multiple times a week, or miserable, resentful duty sex. What you are describing is something in between the two, and it isn't something I've really seen discussed here before.
Often women in our position are told that our relationships, and indeed our entire existence, must somehow be grey and empty and sad. The implication is that we are misfunctioning, and that it is a problem that must be fixed, either through HRT, diet and exercise, or by doing the decent thing and leaving our wonderful husbands so that they can find new partners who want nothing more but to shag them three times a day in perpetuity. I disagree, that it seems that many other women feel the same, given the current poll results.
The only thing I don't fully understand is that I do almost without exception enjoy it once I get going. I have no idea why I am so completely apathetic about the idea of it. I completely relate to your sense of ticking it off the list for the week!