As an aging (50-something) fitness nazi, the key to this is having several different activities of varying impact levels that you can use to attain high intensi

As an aging (50-something) fitness nazi, the key to this is having several different activities of varying impact levels that you can use to attain high intensity. Spinning classes, running, swimming, HIIT / Crossfit derivatives (man be careful with that though), or the elliptical machines. Even aqua aerobics can work well. And I don't think you need a 40 minute zone 4 workout. You can probably do intervals in zone 4 with as much recovery as you need over 20 minutes and induce a lot of (beneficial) stress. Interval training is the key to really good fitness, especially on limited time budgets.