For me it all boils down to one rule: if you don't feel like train-simming, then don't. It's supposed to be fun and if you're finding it starting to drag then go do something else for a while. A break can be healthy and you'll feel refreshed when/if you decide to return.
There have been some good suggestions for other activities here. I'll second them all, especially the ones that involve turning off the computer, which can itself be part of the burnout. Read a book, hop on a bike, go to the beach, etc.
Regards,
Rob.