Cycling
Low impact, precise control, high ceiling.
Control the dose exactly
A bike lets you set the effort with a precision walking can't match: resistance and cadence give you two dials, and the effort stays exactly where you put it. That makes cycling the easiest way to hold a true steady state.
Road or trainer, either works
Outdoors brings variety, a trainer brings consistency. For LISS purposes the trainer often wins, since traffic lights and hills keep breaking the steady effort that's the whole point.
Cadence 80 to 95
A smooth spin beats grinding a heavy gear.
Watch effort, not speed
Heart rate or feel, not the speedometer, decides the intensity.
Great on rest days
Easy spinning helps sore legs recover rather than digging them deeper.