It is probably a good idea to expect that your exercise tolerance will be lower for a bit, even with adequate salt, fat and food. Your body needs to, not only move from carbs to fat for most of its energy, it needs to get GOOD at doing that. And, as CityGirl said, that can take longer for some of us.

Rather than fearing exercise, maybe use it to gather information. Today I can walk X distance without feeling exhausted. What will it be tomorrow?

Keep track; get a little notebook, keep notes on your phone, whatever. But as your metabolism becomes more efficient at burning fat, and you adapt your muscles to the new way of working, you'll be patting yourself on the back for the increase in your exercise potential!

There's a guy on this forum whose forum name is Grav. He's gone from trudging short distances to nearly race walking to and from his work.