So I am getting my training schedule figured out for the winter. I have my LTHR calculated at 175 bpm. I am getting my base work started and noticed that I have trouble maintaining my tempo heart rate on the trainer. It seems a lot more fatiguing inside than outside. My muscles just tire quickly. I can ride in the 150+ bmp range for hours outside but is a lot more difficult to maintain inside. Do I just need to HTFU or what?