It is possible to spend all your waking moments on work. If you define work correctly, that is. As mentioned earlier, countless hours spent on mindless tasks lead to burning out rather quickly. However, if work means working towards your goals and being productive, then it all boils down to assessing the productivity factor in the things you do. That is, if you can see, how every little thing you do during your day gets you closer to your goals, and throw out the things that don't, it is entirely possible. And that doesn't mean you have to sit behind your computer all day creating websites, reports or whatever your profession might be. For example, going out for party can be reconnecting / maintaining your social network, reloading yourself to have more energy for tackling oncoming tasks, rewarding yourself to build motivation for ongoing productivity, upgrading your communication skills, learning to dance, or tied to any other goal you might think of.
One other thing to keep in mind - you must have variety. You said "mostly mental, sometimes physical work". Just make sure you do not forget the physical part. One hour in the gym in the morning is definitely not enough an excuse to spend the rest of the hours sitting next to a computer, the activities have to vary throughout the day. Just get up and run up and down the stairs.
However, dividing it to mental / physical work might not be enough. I am going to give you a link to a blog article, that many of you might have already read:
Seven Ways to Live a More Balanced Life by Scott H Young. This article describes the different "muscles", so to speak, that you are using in different areas of your life. You just have to make sure that you use as many of them on a regular basis as possible and not burn out any single one of them.