This is an unsustainable workload. It's not possible to work productively at this level for anything but the short term. Honestly, for how many hours of the day are you actually productive? Could you have worked more efficently and left after 8 hours instead of 10 or 12? As a cheeky example, if you've got time to read HN at work, could you cut that and similar time wasting activities out completely so that "work time" is limited to just work and get out of the door once the real work is done?

WRT finding time to exercise, I go to the gym for an hour over lunch: 10 mins running, 30 mins doing weights and using various machines and another 10 mins running to wind down, then shower and back to work for the afternoon. Other days I go to the climbing wall and have an hour on the bouldering wall. Then back to the desk.

I also usually cycle to work. The physical exercise makes me feel overall much better both physically and mentally. I can really tell the difference if I don't do anything for a few days. Physical exercise definitely reduces my stress.