Very large couch potato

The best (and possibly only) way to get out of the "couch potato" mindset is to get up and exercise. If you make yourself get up and do something enough days in a row, it becomes a habit (just like being a potato became a habit). There's not a magical formula for this (unfortunately). You simply need to make yourself do it. It gets easier eventually. I rarely miss gym days anymore, and there's always a good reason if I do (ie the gym is closed).

One thing I find very useful in trying to making something a habit is to try to organize things so that you don't need motivation - so that the good option is the default one, the one that requires the least effort. Like having water in a glass already on your desk when you are work, so that you don't have to make a conscious effort to get up and get a glass. If it takes little effort to do something, then when the lack of motivation strikes, it's more likely to still go through with it. For instance, if you want to go swimming the next day, make sure to get ready the night before - for instance, have all the clothes, towels etc. ready and packed, go to sleep early enough so that you're not too tired the next morning. Good luck!