2 miles is 2 miles!! Congrats on that, it's definitely far!!
Running outside can definitely be more challenging, often you are running on concrete which is harder on your legs, also it's not normally a level surface. Running on a treadmill isn't bad, but for me running outside was always a little harder. Keep it up, just because something else might be harder doesn't mean what you are doing is bad. Infact, it's GREAT!! Keep up the great work
-Aimee