My second relapse was a numb left foot, very hard to cope with when your driving a HGV around London.
The third relapse was numbness/pins n needles/ itching legs and the constant tripping, not being able to pick my feet up.
Now secondary is here, I drive short distances. Would not attempt anything more than a hours driving if I can help it.
I think the best, is to have more breaks than normal, and to nap if possible regularly.
I also think that long time thinking effects you, even as a passenger, your still thinking of the pedals/driving.