Why does running on a treadmill hurt/feel harder?

No distractions? How cold is it where you are?

Treadmill is boring and repetitive.. so you prolly notice every little pain/discomfort... Outside there are more distractions.. so you don't notice that stuff as much.. 

My guess. 

Also remember there are big differences between treadmill running and running outside.  For one, when you're on a treadmill your pace, climate, etc. is pretty much constant.  You are always running X mph at X incline no matter what your stride/how you feel unless you consciously change it.  When you run outside, your pace is constantly changing based on how you feel, your terrain, etc. and most of the time you won't even realize it because its automatic.  

The biomechanics of treadmill running is also different.  The ground is moving below you and you.  When you're on the road, you're producing a particular force on the ground to propel yourself forward.  

Know how you manage that? Asking because most people tend to overstride and heel strike more on the treadmill. Think it's the perception that the belt is coming at them, and the tendency to drop hips and absorb it more than drive their leg through. I have heard of and experienced the more sore calf (well, all lower extremity parts you mentioned), but had heard it attributed more to something in the later part of the stride.

Hmm...Maybe because I tend to run faster on the treadmill?  Outside the footing here is not great (that's how I got hurt in the first place), and I do much of my outdoor running before dawn or after dark during the work week, so I might be more cautious. It could be I'm just going faster or running more confidently and thus heel striking less. ETA I set the treadmill speed based on HR zones and it just occurred to me--it's 20 degrees cooler inside than out in the tropical heat, so I'm no doubt able to run considerably faster at the same heart rate/effort level. Duh. I don't have my Garmin here but guessing it's about 30-40 seconds a mile difference.

I know I normally am more on the balls of my feet when doing faster training or racing (can see from pictures of shorter races, or the finish of longer ones), and occasionally have the same soreness then. I do notice it less on the treadmill when running at a very easy pace such as during warmup. I know my footstrike is different at faster speeds because I can actually see as well as feel it--the mills face floor to ceiling windows and at night you can see yourself run. Sometimes I will try to get back to heel striking on an easy treadmill run and if I stop concentrating, I'm back on the balls of my feet again!

^^^WINNER, WINNER, Chichen DINNER

That makes sense. A lot of it is with confidence in the surface you're running on. There are a number of crummy sections of roads here too, cracks and unevenness along the edge where I run. My bodyweight might shift back a little more (meaning a little more heel than midfoot) when I'm on this and it's dark (and if it's wet or chance of ice). Then I'm more centered, so it's easier to catch myself if there is a slip or trip. Same thing happens on more technical trail sections. When I'm feeling more confident in it, I'm able to be more forward and up on the mid or front foot.

On a treadmill, the time and distance seem to go by SO SLOWLY because I can stare at the numbers the whole time. When I'm running on the road, I may check the Garmin every so often but I don't actually watch it going 1.10....1.11.....1.12........ On the road, if it's a familiar route I'll know what house or road marks 1 mile or 2 miles or whatever from my house and it's more interesting to run to that house rather than watching every hundredth of a mile....or I'll just zone out and not know how far I've gone until I look at the Garmin and realize I just ran a mile and it felt like nothing.

I've tried covering it up with a towel but I just yank the towel off and then get disappointed with how far I HAVEN'T gone yet.

It depends on which part of the running on a treadmill you consider hard.

Treadmill:  physically easy, mentally hard
Outside:  physically hard, mentally easy 

What I find is that while I get a fantastic cardio workout when running on a treadmill, it feels almost as low impact as riding my bike.  When I get done, I have sweated a lot and had a great HR profile for the workout.  However, my leg muscles don't seem to have been worked out as well as when running outside.  Also, running outside stresses all sorts of important things (in a good way - helps to build them up) such as ankles, calves, tendons, ligaments, etc.  I don't really feel like I get that when running on a treadmill.

So in short, I find running on a treadmill physically easier and lower impact than running outside.  Mentally however, running outside is easier.

x2.  Anytime I'm being "forced" to hold a pace that I'm not setting myself it makes it feel harder...