Had a good couple of hard sessions down local boxing gym, Spit n Sawdust, recently.
We do a mixture of strength and conditioning exercises and boxing work. It pushes me really hard and focusses on the same sorts of muscles used when driving the race car - upper body (shoulders, neck and chest), arms, back and core.
Once the F2 season is over and depending on what driving I have lined up for the winter period we'll begin to shift the emphasis towards strength and away from pure endurance for a few months.