1. Elliptical cross-trainer
A great alternative to a run as you can keep your pace (and heart rate) slow and steady, or you can mix it up with some interval training to get your heart pumping.
Remember to keep your heels down to engage your posterior muscle chain. Avoid using the outside moving handles as they don’t provide any benefit. Lightly grasp the narrow fixed handles instead or drive your arms as if you were running.
2. Rowing machine
Perhaps the best bit of equipment to practice bursts of interval training. Set yourself a distance (250m, 500m, or even 1km if you're feeling brave) and do it at threshold pace.
Rest, then repeat to improve your speed endurance.
A high-intensity spin class is a fantastic way to mix up your training, spike your heart rate and help your prepare for high intensity runs.
Invest in a pair of cycling shoes/cleats so you can clip into the pedals. This will allow you to pedal in an efficient way and correctly pull up and push down during each pedal stroke.
Try a swim in your local pool to rest your joints and still get all the cardio benefits of exercise.
Mix up your strokes and speeds and take a breather when you need to! Avoid kicking your legs hard all the time as this will tire them out quickly.