Six weeks ago, I couldn’t climb two flights of stairs without stopping to catch my breath. That’s a hard thing to admit when you consider yourself an active person. I walk 8,000 steps a day. I do strength training three times a week. But stairs? Stairs were quietly exposing a gap in my fitness that I’d been ignoring. So I decided to do something about it — not with some brutal program, but with a simple, progressive approach that anyone over 60 can follow. Why Stairs Are the Ultimate Fitness Test After 60 Stair climbing combines three things that matter as we age: leg strength, cardiovascular endurance, and balance. It’s…