Ultimately it's nudging your comfort zone from where it is now, whatever that is now.
I posted about my motivations regarding arthritis a few pages upthread. My logic is the better that I maintain my muscle and bone density in my 40s and going forwards, the more I have to work with as I age.
I am fit. Objectively and for my age. But that's been earned by prioritising it.
I had SPD when pregnant to the point of being offered a mobility scooter as I hauled my heavily pregnant body through the supermarket car park on a pair of crutches (I politely declined as I was hoping the effort might trigger labour by that stage 😂) That was an improvement on first time around where I was fobbed off about my "pregnancy aches and pains" and couldn't leave the house independently in the last month. Second time round, the SPD lingered post-natally. With the help of an ostepath and following the rehab exercises I managed to address the ongoing pain. I then realised that I was pain-free but limping. 9 months of constant pain had changed my gait and it was a habit and with a couple of weeks of very conciously trying to even my gait, I was walking normally. I then took up C25k because I was fed up of panting around. A year later I did a HM and was in love with running.
I had the advantages of being early 30s and the condition being recoverable, but I chose to work with the pain and improve my health. I literally started with 100m walks with walking poles and going to bed to recover (so that took more out of me than running a marathon!) and then 15 min pilates videos. I chose to keep it up when I went back to work using late nights/ early mornings, and the weekends for longer sessions. I'm in a good position now but those early stages were physically hard, and logistically hard. It was worth it though because my body is stronger and faster now than 20 years ago.
Every step along that way has been listening to my body and nudging its capabilities a bit further. The outcome looks very sporty today, but those foundations were very modest
I've had relatives die prematurely from lifestyle related conditions. I've seen relatives have long lives with long, frail old-age stages. I'm mortal, but I'm hoping that looking after my body now maximises quality of life for as long as is feasible. If nothing else, "using it" is fun and interesting in there here and now than minimal activity levels (despite my PE teachers' best efforts at convincing me that I was lazy and entirely non-sporty)
Progress is usually just doing a little bit more from where you're at. Sometimes you get new results, sometimes it's maintaining where you are.