I too have tried running on the treadmill in the winter months as it can be -40 here in the winter. I kind of figured out the mechanics are a little different as on the treadmill, you basically just have to lift your legs so to not move backwards and on the road you have to push off to make yourself go forward which takes more energy. I think that an increase in distance on the treadmill will help but I still find when I hit the road for the first time in a while it is always difficult.

Don't get discouraged - you are not alone!  I could have written your message myself.  Pretty much the same thing happened to me, as far as the heat thing goes.  I began running last year, doing both treadmill and outside (walk and run) and progressively got better, have run several 5Ks and felt so good about my times getting better in each progressive 5K, as well as feeling stronger and running farther in my personal runs.  Being a 53 year old who is a novice, I don't expect to run a 5K in 30 minutes, but I have been quite happy with my progress for ME (35-37 minute 5K times).  Well, I live in Florida and when the hot weather got here, it was like I hit a wall!  I didn't get out of breath at all, but my legs just felt like jelly and so tired and weak that I had to walk the last mile and a half to two miles during my 5K runs, and I felt so miserable (overheated and nauseous).  In April I ran two 5Ks with times of 40+ and then 42+ minutes, same thing 7/5 - 42:56.  I was SO discouraged and disappointed, and felt like such a loser.  Then I realized that I am just not a hot weather runner and to stop beating myself up over it!  I just do not tolerate hot weather for exercise, and there's nothing I can do about it.  I'm not going to do any more 5Ks till the weather cools off, and I'm just going to continue doing the treadmill (I do 30-45 minutes of walk/run every other night) and also long, brisk walks outside, with a run now and then in the early a.m. before the sun is up.  So please don't give up - and keep at it!  I bet you will do better once the cool weather is back.  I am determined to do better, too!

As Urbster said, generally you need a 1% incline on the treadmill to make it equivalent to running on the road under ideal conditions. (chart here: http://www.trinewbies.com/run_treadmill_chart.asp ).  BUT . . . 90 degrees plus humidity is far from ideal conditions and far from running in air-conditioned comfort inside.  It's very simple, and very hard: you have to slow down.  In those kind of conditions you can expect to run 1 to 2 minutes per mile slower.  Just learning to pace yourself in hot outdoor weather will take some time.  Don't get discouraged, don't trash your shoes.  Run slow, adjust to the heat, and you will start to feel better about it.  Honestly, I feel the same way the first couple of weeks in the summer, like a slug.  When the weather gets cool again, you'll find it easy to pick up the pace.

Len

Just a comment on this issue as well.  Yes, the warmer, more humid air plays a role.  Far more important is your body's need to cool itself.  The blood supply must feed your muscles for you to run.  But as you get hot more blood is routed to the skin in order to cool the body.  It can't do both and the preference is to cool your body (ie. keep you alive), forcing you to slow down.  Running slower to begin with helps reach a better balance.  You won't get as warm, so there is less need for cooling, leaving (hopefully) an adequate blood supply for the muscles.

Here's a real-life example.  I ran a 10 mile race at the beginning of May, in 60 - 65 degree weather.  I averaged 9:15 for the last seven miles (the first 3 were slower because of a physical problem.)  In mid-July I ran an 8.4 mile race in 82 degrees and averaged 10:30.  That's a 1:15 per mile difference for a 20 degree difference in temperature. 

Len

Here's me (575)  the one with the red face waiting for death to come for me and trying my hardest to catch that 8 yr old  who is skipping along without a care in the world.   I think those other two people are his parents just making sure I didn't knock him to the ground because that was the only way I could have beaten the little scallywag.    Yep we're in the same boat.   In the last two months I've improved my time and no longer feel the grim reepers breath on my neck.   Stick with it, give yourself a few months and you'll see. 

And  I'm telling you right now- one day I'm going to beat that kid!!

Too funny.   And what is funnier, I'm doing a 5k Ice Cream run this Sunday!!!  I'm sure he'll be there.   Maybe I can load him up on dairy proir to the race.

"Is there a difference in performance on a treadmill and outdoor running?"

Are you kidding?  You know the answer to this question.  You are living proof!  I know you are down on yourself right now about the difference you've felt physically, but let's look at what you've accomplished:  You went from not being able to run 100yds to being able to run 6 miles at a stint.  Huge accomplishment #1.  Again, you went from not being able to run 100yds to doing 4x5 mile runs a week.  Another huge step.  You reduced your running pace from 15 min/mile to 11min/mile.  If I could cut my per mile time by 4 minutes, I'd celebrate like it was 1999!  And, you lost 70lbs all while doing this!  That has to feel wonderful!  Let's think about the health benefits of that...less pressure and strain on your heart and the the organs your vessels supply.  You probably reduced your cholesterol numbers.  Have you had blood work done lately?  Do it and compare with former bloodwork before you began running.  You've probably increased your natural life span by a number of years.  All that said, I do hear your frustration.  I agree with your husband.  I think it's the air your breathing.  Have you tried to baby-step some training to ween you to running outside again?  I've found that when winter turns to spring, and spring turns to summer, I need about 2 wks to adjust (while running outside during those two weeks) to the new weather.  I just ran for a week in Bermuda (I live in CT) and took a couple days to adjust when starting to run back in CT.  Give yourself time.  Also, I've used an inhaler in the past for exercise induced asthma.  May-be ask your M.D. about that option. 

Boy, you are stubborn.  I just read the last line of your email.  Re-read it again picturing someone else saying it, and see if you don't find a little stubbornness in it. 

Run outside for about 2 wks to a month and be aware of how your breathing feels during every step.  Log it so you can look back on it.  Don't throw away your shoes.  You just need some outside time.  GET OFF THAT TREADMILL except for the coldest winter months.