Is walking enough, or do we need more cardio after 45?

I love my daily walks and I'm not willing to give them up, but I sometimes wonder whether they're doing enough for my heart health now that I'm in my fifties. I've read conflicting things and I suspect many of us are in the same place — loyal to walking but a little uncertain about whether to add something else. Do you rely mainly on walking, or have you found a cardio mix that works well for you at this stage of life?