I live in Buffalo and I find that my tempos and base running tend to go well during deep winter. Time becomes meaningless so I have to go by effort. I didn't realize until I typed that just now, but that sounds a lot like what Colorado deal with given the combination of altitude and big climbs; you don't get to roast your tempos, you have to go by feel and learn your body and get aerobically strong. I can run some faster intervals on a treadmill if I really need to, but mostly winter is a base season anyway, and you don't need much to stay in touch with your speed. I think that is kind of a blessing in disguise.