I agree with you about exercise lightening one’s mood. It is hard for me to get out the door for a run, but I am always glad it did it!!
I learned a long time ago to trick my body and mind into exercising.
If I start thinking negatively about it, I tell myself to just go for a walk or easy run or easy bike ride or row. Maybe even just a short one.
Once I get started, I always get into the swing of it...and, as you say, am always glad I did.