Do not increase the distance, and as for incline, I personally don’t and some research has shown this only matters for people running at 7mpm or faster. And one suggested 3-4%——-that is Boston Marathon hill training incline! (Really, that is exactly the plan I am doing now.) The difference in running inside vs outside could be due to many things. Is your pace the same? TM pace measurements can be off vs outside. Also, outdoor roads or sidewalks are less even and possibly more cambered, so you engage you foot and leg muscles differently, and the road surface is definitely harder with asphalt or concrete vs a TM belt.
I wouldn’t worry too much about it. I used to feel the same way. Over time,as I ran farther and built some endurance, the feeling went away. Now, I tend to run a pace outside that naturally falls faster than what my easy runs should be, and I have to work to slow down!