When do adverse conditions outdoors hinder our training?

I live and train at 8500′ in Colorado and I have had this question for YEARS….when do our training sessions outdoors build more character than endurance, and is it ever detrimental to our progression?

Here in at our elevation in CO, the trails are icy, slick, or postholey for 4-5 months a year. It can make running slow and frustrating, because I feel like many times the conditions are slowing me down enough that I’m not actually allowed to get faster or improve because I’m having to be too careful. And its not just a winter issue, but also the rest of the year in inclement weather, or on terrain where we have to be mindful or our steps and slow down to to a pace that puts me far below AeT.

I have a treadmill and I dont hate it…but I also dont feel comfortable relying on it for really building the strength and conditioning I need for mountain running. I use it really only if I cant get my workout in until way after dark, or if the weather/trail conditions are completely horrible.

Is it always better to be outside training on a trail, even if we’re moving slow/below AeT, or should I be be running inside more often when trail conditions slow me down? Currently I’m training for the Leadville Marathon, so both being able to move fast enough to develop my speed is seemingly as important as conditioning my body for the mountain terrain….but these icy crappy trails right now sure aren’t making me fast! Ha!