Great point. I like to compare work capacity to exercise capacity. Too much exercise will actually harm your fitness rather than improve it. However, reasonable exertions, combined with appropriate rest, will improve your fitness over time.
Like exercise capacity, one's ability to work hard can also be improved through practice. This doesn't mean pulling all-nighters and chugging caffeine to override the sleepiness, though. It means setting incremental goals to try a little bit harder and then following up with proper rest and recovery.
For example, if you install time tracking software and measure that you spend 3 hours in your code editor every day (a reasonable amount for someone working an 8-hour day, due to time spent reading documentation, in meetings, and other activities), it would be a mistake to set a goal to spend 6 hours in your code editor. You'll get burned out and hate it.
However, if you set a goal to spend 3.5 hours in your code editor every day, you can likely find low-impact ways to make that happen. Maybe you're more efficient with transitioning from meetings back to coding. Or maybe you cut down time spent reading articles on HN or Twitter by 30 minutes and apply it to coding instead.
Over the course of a 5-day work week, that extra 30 minutes per day adds up to almost 3 hours extra work. If your starting point was 3 hours per day, you've basically added an extra work day to your week without giving up much.