I have been trying to get my 1.5mile run time down to 10 minutes or below. To do so, I've been training both outdoor and on the treadmill, and have been encountering problems. When I run outside, I can keep a comfortable speed and finish below/around the 10 minutes mark. However, when I try this on the treadmill (keeping an average speed of 14km) I find it near impossible to sustain the speed for the duration of the distance, as it feels I am sprinting throughout the run. My question is, although there are noticeable differences between outdoor running and treadmill running, why is it that I am finding it near impossible to replicate my outdoor times by a large margin?