For what it's worth, I know a lot of full time folks who only really work 4 hours/day. The other 4 hours are spent in meetings, talking to coworkers, browsing the web, learning new things, resting the brain but in the office so they "look productive." So I wouldn't feel the need to be productive 8 hours every day.
I wish you the best with taking care of your health! So important indeed.