To jump around in the pool for 45 minutes doing aqua-aerobics, which is the most exercise I ever get nowadays, uses up approx 305 calories. Given that a packet of Walker's crisps is 184 calories, that tends to put the whole thing into perspective. People eat crisps and hardly notice that they've had anything.
As for cake, the main problem is the sugar content. Sugar is addictive and it produces an immediate spike in blood sugar and spike in insulin production with a crash soon afterwards. All the up-and-down is not good news.
How to stop eating cake? Don't go anywhere near it! I grew up learning how to make cakes, buns, scones, pastry, any flavour or variety, all those things, but I don't any more. Last week at church we had some iced buns (cup cakes?) thrust under our noses on the way out but we sidestepped and escaped very neatly. Someone had been busily and lovingly making and decorating them, but we didn't any. Otherwise, we don't go where there are any.