Pros race with 11-27 53-39, climb the Alps and The Pyrenees Mountains.
maybe you need to get the body into power shape ?
I'm more a Tourist with a Road Bike.. its got a triple crank , I neither run or swim much.
Id buy a different chain to stay with each cassette , since they will wear into each other
and replacing just the cassette can cause chain's skipping .
quick links and the Park pliers for the purpose.. make it easy to open ..