You're doing very well. I've noticed that cold and snow on the ground tend to cause the run to slow down. I did one off the record in 4" of snow and got a time that was nearly a third again as long as it took at a more reasonable temperature. Extreme heat seems to also have a similar effect. I've also noticed that working out on treadmills isn't the same as running on a track or road and that I have to speed up on the treadmill to get the effect of my normal track pace. I don't know why these are just things I've observed in my workouts. YMMV