i don't agree with the treadmill runs simply because the measured progress is deceptive. yeah, it gets boring but it's basically 'running for you'. you don't have a choice whether you want to put one foot in front of the other because the machine projects you forward. outside you have to produce your own momentum, making it more difficult. ever try HIIT on a tread v. outside? significantly different animals and MUCH more taxing outdoors.
not to mention, you won't be tested on a tread so why train on one unless weather isn't permitting?