Since your subject heading is "sore calves" it sounds like your calves are needing a rest day here and there to recover. Later on, as you get more conditioned, you may be able to climb stairs longer and/or more often than you are currently.
In my 30's I was doing step aerobics daily....I started getting tight and sore calves. My body was telling me something...unfortunately...I wasn't listening. My calve muscles sustained an injury that I could have avoided.
Cross training is a good thing, allowing you to continue to exercise but avoid repetitive stress to the same muscles. Try another activity for one or more of those days, and if you are not already, consider an active rest day.