Well, I take it back my energy must be returning. I managed a Shiva Rea Heart-Air flow, which isn't fast but has lots of backends and twists, then added a quick forward bend flow, as my back feels a lot better but it's moved into my hip, so wanted a good stretch. It felt good too, I was happy to manage a vinyasa flow without giving up part way.
AugustRose and others, I agree about yoga making you feel good, I really find it sets me up for the day. On a superficial level, it rid me of any spare tyre round the middle too (though, actually, that is beneficial to my health as well!).