It sounds like she's in the negative feedback loop of not working out. If she hasn't worked out for a while, starting a new routine can seem really intimidating, and it's easier to make excuses and keep things the way they are. A few days of cardio and quality workout should get her endorphins back up, and back on the right track.

Has she seemed depressed/sluggish about any other areas of her life? Work/life balance, family problems, everyday stress, etc? The reluctance and blame shifting might be a signal of a deeper self esteem issue, and she may just need assurance and a helping hand.

Working out, walking, biking, doing anything physical with your SO will make the two of you happier and stronger, so I hope it works out.