A hopping kangaroo lands with its ankle joint flexing under load, stretching the long tendons that run down the back of each hind leg. Those tendons behave like springs: they store strain energy on landing and return much of it on take-off, so muscle does less work per hop than a naive accounting would predict.
Respirometry work on kangaroos hopping on treadmills found that oxygen consumption rose steeply with speed while walking on all fours, then levelled off once the animal switched to bipedal hopping. Faster hopping is achieved largely by taking longer hops rather than more of them, so stride frequency and the breathing cycle coupled to it change little. Red kangaroos cruise comfortably at roughly 20 to 25 kilometres per hour and can sprint far faster in short bursts.
The common overstatement is that hopping is free. It is not: the elastic return is partial, and at slow speeds hopping is actually awkward and expensive, which is why grazing kangaroos use a slow pentapedal gait with the tail as a fifth limb instead.