Does being fit mean being thin?

No, recent research has discovered that thin people (excluding those with eating disorders) just have a genetic difference from those people, eating the same amount and exercising the same amount, who put on fat. Being fit, means not getting out of breath when trotting upstairs or climbine a moderate hill.

It means feeling able to undertake a walk of a few miles without being exhausted or bending down and stretching up without having painful twinges. Daily exercise keeps you fit, of whatever kind.