Go ahead and do incline, rather than speed, for now. And consider mixing things up.
Maybe one day gradually go up higher and higher every few minutes...
Another day, do interval hills...flat/hill/flat/higher hill/flat/even HIGHER hill
From time to time, you might also want to play around with speed...don't go crazy, but add a tenth or 2 tenths every couple of minutes and then alternate with the pace that you're comfortable at. I never thought my legs would move fast enough to go beyond 3.5, but now even 4.5 is still a 'walk' for me.
As your legs get stronger from walking the inclines, you'll find that you're more agile, too, and that will help you move faster.
Regarding the inclines...whatever you do, don't go so high that you have to hold onto the treadmill. That significantly reduces the number of calories you'll burn.