There's more than code to build a startup. I have found the best way for me to avoid burnout or alternatively feeling like i am behind is to set very high goals. Find a lifetime goal. Maybe it's making a startup, maybe it's something else. Everything you do should mostly fit into some super long term plan that could take an entire lifetime's worth of work. If you are doing creative or personal projects, these build skill and experience which will be needed by that long term project.
Martial artists train by punching trees. To build micro fractures in the hand bones and increase the density and strength of these bones. Go punch your trees.