I'm not sure this is the best advice. Providing more flexibility allows for the spontaneity of motivation that can help you escape a rut which is what the OP is

I'm not sure this is the best advice. Providing more flexibility allows for the spontaneity of motivation that can help you escape a rut which is what the OP is asking. This assumes that you can be instantly productive in the 3hr work blocks. If you can't get into a flow for the first hour, then you only have 2 hours, etc. And you start to look forward to your breaks. Flow works best when you have open-ended working sessions. If you are making good progress - don't stop it for lunch. Grab something quick, and then have a larger dinner.