Walking is fine! Anything that gets your heart rate up will work.

If it gets *really* narrow, that's when you can get shortness of breath and possibly a heart attack. Your heart isn't delivering enough blood (oxygen) to the muscles, so physical exertion gets difficult. My brother had a mild heart attack, and he had a 90% blockage or something like that. They gave him a stent, and now he goes to the gym 3x a week and does plenty of aerobic work.

I have a 40% narrowing in one artery, and I can go jogging for 45 minutes and I feel perfectly fine. I could easily go longer except my knee would act up. 🙂 My cardiologist told me "keep running."