If you want to work out in a walnut-floored boutique studio with an exclusive handful of classmates dripping in Lululemon finery, you must be prepared to pay for the privilege. And it seems that we are. Our fitness expenditure is spiralling, not just in terms of the amount paid for bespoke classes and personal trainers, but on the high-end clothing we wear to them, the £200 running shoes and the entry fees for ever more glamorous endurance challenges.