I’ve been getting about half my weekly aerobic volume from hiking hill laps, unladen. Walk up, walk down. HR is quite comfortably in Z1/Z2 on the way up, sometimes drops a little bit on the way down but I don’t worry about that. Less that 15% grade gets a bit inefficient and the one time I used stairs it beat me up noticeably worse than when I used foot trails, but in general it’s been very low impact and it’s obviously more specific to foot-borne activities than rowing, riding, or poling.
Too soon to tell whether this is working, and I won’t claim it doesn’t feel a bit silly to put on shorts and shoes and go walk, but this is the advice I got more or less verbatim from the coaches.