It feels harder to me, but I think it's because of the lack of airflow, the boredom factor, and when I run outside, it seems my pace is constantly changing (I have my Garmin set to instant pace).  I think it is really hard for my body to run at a constant pace.  I do think that if you plan to race a lot the best thing to do is to train under the types of conditions you'll be racing on (road, trail, etc.), but that's just my opinion.

The mill is what it is.  There are some Olympic caliber athletes who have done a lot of training on treadmills.  I read about one who sets it up facing a concrete wall and doesn't wear headphones in an effort to build mental toughness.  Personally, I find it more difficult to run my normal training paces on a treadmill but easier to do speed work.  Go figure.

Lily is a beast on the TM and runs fast in races, so it works for her.

I've always said TM running is not the same FOR ME.  It's usually 80-100F and 76-80 dewpoint outside for 9 months of the year.  The TM is not.  Which is why I run faster on the TM.  Also, my doctor has recommended I train outside because training on the TM will not prepare my asthma and my lethal allergy for running outside later.  The more I run outside, the more antibodies I can create for the allergy, etc.

If it works for you, it's a great tool.  If you cannot run outside, it's a good substitute.  But for me, it's not the same (and I know because I used to train on the TM for years).

I wouldn't be surprised if the next thing they tell us is that TM running gives cancer...

I just assume everything does.    If you're feeling sick or injured, read enough Google links and eventually you'll be convinced that you have cancer. 

The best part of TM running is you have all the control, ironically, that is also the worst part.  I have run a little over 2600 miles this year and about 800 of them have been on the TM. My longest run is 20 miles, a little over 3 hours and I like to do speedwork on the TM.   I personally don't find it any different than the crushed limestone rail trail I run most of my miles on but I do find running on the road faster than both.

BTW-DoppleBock is insane and also a great beer.  I would highly recommend Capital Brewery Fire, which is a Dopplebock, best beer ever.

I'm slower on my treadmill... Have a much harder time maintaining my regular pace.  It's strange.

Same here.  Outside, 10:20/mile is my long run pace.  On the treadmill, it feels too fast to be my easy pace some days.  

I figure there are a few things - first, like the article said, it's HOT in my gym.  They have the temperature set to like, 70 degrees.  Then everyone works out in there and creates a ton of heat.  And on top of that, the cardio equipment is on the second floor, which is open to the first floor lobby area.  So they pump out excessive amounts of heat so that the lobby area stays warm when the door is constantly opening....and it all rises up to the treadmills.  Awesome.  When someone lingers and holds the door open too long, a cold breeze wafts up periodically, and it is HEAVEN.

Second, I think that the treadmill does something to my gait.  My legs just feel like they are forced to move faster.  Outside, 11 minute miles are my stupid easy recovery pace.  I feel like I am moving my legs in slow motion.  On a treadmill that feels easy, but like I have to keep up a much higher cadence to maintain it.  Part of it is that I think I have too much of a subconscious fear of falling off the back, and I'm constantly brushing the bar in the front.  I wonder how much momentum I lose each step by essentially running into that bar.

  Personally, I find it more difficult to run my normal training paces on a treadmill but easier to do speed work.  Go figure.

Likewise.

You know what though? It doesn't matter. I've done quite a few 20 milers on the treadmill. And quite a few outside. With all the other variables involved, who knows if I would be a pound lighter or heavier or a minute faster or slower if I had done them all inside or outside?