I'm 34 now, and let's be real, sometimes the same routines day in and day out get real old (I used to run 3-5 miles a day... everyday...), so I'm compiling my list of treadmill exercises I use to challenge myself and mix things up and I'm bringing them to you just in case your runs start to get boring or you get the urge to want to save your knees, as all of my routines are done on hills/inclines.
I have labeled them all beginner/intermediate/expert for your convenience.
Disclaimer: I am not a doctor by any means. Consult your physician if you're unsure of any routines or have any health conditions. I do, however, come from a strong background in health, fitness, & nutrition and this is simply my advice.