Find the number of steps that works best for you. The number may change from day to day. Try to walk a little father each day, even if it's only a few feet. But don't push yourself too hard. If you become short of breath, stop and rest and practice pursed-lip breathing.
Some people carry a portable camp stool in case they want to sit down along the way.
Walking and other exercise use many muscles. As you work them, they slowly become able to do more work with less oxygen. That means you don't have to breathe in as much air to do the same amount of work. You're stronger and can do more before you feel tired. Walking is a good exercise to begin with, but think about other exercise or activities you enjoy.