There will inevitably be times when a runner is injured or perhaps needs a joint replacement and he or she wants to maintain endurance fitness while recovering. This can be done by cross-training, substituting a low or non-impact form of aerobic training that can be done without stressing joints and causing pain. Here are the possibilities:
If any one or more of these is done fairly intensely (e.g. intervals, race pace) and for the same length of time you would normally run, there is no doubt you can maintain your stamina till able to run again!