Why do you think my usual exercise routine feels so much harder?

Very much could be related to your SAD. I pretty much fail to workout at all during winter, it's brutal on my mood and energy.

Go for a walk that usual works for me.

How far/fast are you running.

If you're grinding out the exact same distance at the exact same pace, you're going to hit a point where you reach equilibrium or worse, begin to backslide as your body adapts to the training.

So, IF possible get outside. Vary your distance and pace... it doesn't have to be much, but mixing it up will keep you progressing. If your target is 3 miles/30 minutes. do 5 min walk warmup/cooldown and a 20 minute faster than usual run... or push to 35/40 minutes with a little more distance and a little slower pace.

Absolutely could be your SAD. What are you currently doing to manage your symptoms?

I've ultimately found an anti-depressant to be the most helpful - it gets me about 2/3 of the way to my usual self. From there, it's various self-care things, like a consistent bedtime routine, yoga at night, some sun when possible.

January is always tough for me.

~Lyssa

Could be depression. Could be your TM is not calibrated correctly. I was having real problems on mine. Running my normal pace felt really really difficult. Turned out it was stuck at a higher incline than I thought, so it was actually more difficult. By zeroing it out, I was able to run much more easily. Another possibility is ferritin/anemia or vitamin D. A few years ago running became really difficult. All my runs were a struggle. I was tested for anemia and came up normal, but I asked for a ferritin test and it was really low. I started taking iron supplements and within a month running was fun again.