Yesterday I broke a 40-hour fast at lunch, then exercised around 6 hours later.
I definitely didn't feel strong/energetic - but my numbers didn't actually suffer.
That being said, it was a relatively short fast that started when I was glycogen-saturated. A longer fast that starts in ketosis might be different - which is what I'm planning to do next week (60 hours after 5 days of < 20g carbs).
The first 24 hours breezed by, but it started getting tough around hour 42. Note this wasn't hunger - I actually wasn't hungry at all. The problem was overall lethargy and feeling foggy/light-headed. I did bounce back after a few hours though, and although I felt very weak around hour 58, mentally I felt fine. Constantly felt very cold, too.
I don't think the lethargy/weakness was due to dehydration (though drinking a lot of water on the second morning helped) or lack of sodium (was ingesting salt regularly). It could be lack of other minerals (potassium/magnesium).
The worst is that it really interferes with my sleep. It takes me hours to get to sleep and then I wake up every couple of hours.
I actually think I'm going to stop fasting for this very reason - it's not like I was doing this for weight loss, I was just experimenting to see how it felt. OMAD/IF will probably give me most of the health benefits of fasting without any of the drawbacks.