I get my HIIT in cycling. It's low impact on older knees. It's not hard to get in 3-5 hours in intermittent all-out effort. Combine that with some kettle bell lifting and calisthenics with yoga on off days to stay limber. It's about the best I can do for an older body.
Just as an FYI, dragging a weight like a big log backwards is a great leg exercise that is much safer than squats, especially for older folks.
