Okay, great, so it sounds like there's a little bit of flexibility. I'd try to build time in the calendar for walking - whether that's at morning/evening "commute" time, or by blocking some time off on the calendar (I know, I know, way easier said than done). It took me a long time WFH before I started blocking off my calendar for things like lunch, but you're entitled to that break during the day, so you should take it.

Maybe try something like an under-desk bicycle or a treadmill desk to try and build a little bit of physical activity into the day - I find myself on meetings every once in a while where I'm just listening in or with my team who knows me well, and I pull my bike trainer up to my desk and ride on Zwift during the meeting.