I understand where you are coming from there. I'm from the southern part of the U.S., and summers can be unbearable on a good day. Treadmills do propel you forward and have a bit more give when you run on them. If you are training for a race sometime soon, then it may help to run outside as much as you can to get used to the terrain and weather. If you're not and using this as an exercise program, you should be fine. I wouldn't worry about losing progress for the two weeks you need to use it. It may even help with speed and maintaining a steady pace. I know I have issues when running outside making sure I'm keeping my pace constant.