How do incline vs speed vs duration affect the quality of a treadmill workout?

Spending 30 mins on the treadmill is good given that you are incorporating interval training i.e a combination of running and walking, or running all the way if you have the stamina. Incline requires more effort thus giving a more intense workout to the calves and hamstrings. They will strengthen over time. Typically you should set your incline to about 1-2% or at max go to 4%. Anything more and you're probably showing off. ;) Do not run or walk on the incline position for too long as it has a detrimental effect on your knees and hips. For 30 mins ideally walk or run slower than your normal speed [the speed when the treadmill is not inclined or flat] for about 2-3 mins in that inclined position and bring it back to the flat position for about 5 mins before again setting it to an incline position for 2-3 mins. Do this about once a week. Do not do incline every time you get on the treadmill. Running at a higher pace at an inclined position will again cause injury. However, your workout shouldn't just consist of 30 minutes on the treadmill. Make sure you do strength training as running alone will make you weaker over time. And as always be patient and listen to your body. Stretch well and drink fluids after your run. Happy Running!