Lunges for Peak Fitness?

Before the New Year, I started my walking routine again. It's basic cardio-fitness, and it gives me an opportunity to pray for the folks in each house that I pass.

I do pushups for upper body strength, and I supplement with self resistance exercises for upper body strength. What I really need, though, is something that will push me to peak output, which has numerous benefits, including fat loss, stimulus of body's natural HGH production, increasing efficiency at a cellular level (mitochondria), and so forth.

I know that sprints are probably the best way to do this, but the last time I tried it, my knees and ankles took such a pounding that I knew I would do damage if I persisted. That's why this video interested me.

The lunges can provide an intense workout without impacting my joints. I can also do them indoors when the weather is bad, as they don't require a lot of space.

The tough part is just making them part of the routine, so that they become a habit. Anyway, here's the video: