Haven't done altitude training, but have run while on ski holidays, staying in a resort height of 2050m.
I used to do a few miles up and down the mountain in the afternoons, shuttling between our resort and the next one down which was 1800m.
It was OK going down, bit of a b*gger coming back up. I didn't find it any different other than I would get more breathless and find my HR higher than when bimbling around at home, but having sorted that out on the first day, had no problems after that. Coming back up was just like doing continuous miles of hills.