"on good/great form" but "in good/great shape"

That's very close to my reading/writing, Jo.

By saying TEd was "on great form" I was talking primarily about wit/performance rather than physique. If I'd said just "on form" I would have been talking about performance in a sport or competition. Less about wit, though it might (just) be a mental challenge at which TEd was excelling.

Of course TEd, being a true Renaissance Man, may also be "in great shape" physically. Starting with the "in" seems to be enough to imply fitness, though, even if you end up with "form".

Quick rule of thumb then -
"on" = wit, "in" = physique

Everybody clear now?