I've recently been diagnosed with bi-polar, and im finding that one of the things that helps me clear my thoughts, is doing cardio. Is it "okay" to do 40 minutes of cardio on the days I don't lift weights? I'm not talking HIIT, just a nice steady pace on the bike or the elliptical. I'm just thinking in regarding to "losing muscle mass", or is there no such thing?

I'm following a good diet plan, so if i burn 300 kcal on the days I do cardio, i'll just make sure i'll eat a bit more on that day.