Training Stairs

I just started my training for spring's climbing season. I decided to stop running (a while ago) as it kills my heel and didn't help hiking anyway. So I've been doing stairs at my work with a pack full of kitty litter (40lbs) and rice (15lbs) and magazines (5ish). According to the scale at work it weighs in at 75lbs. I was going up and down the stairs but switched to taking the elevator down from the 9th floor. I've been doing 60-70 flights and am aiming for 100 now that I'm using the elevator