Injuries can certainly hinder our training routine, but endurance is something you can continue to practice!
Mental endurance can be just as important as physical endurance, and they tend to work hand-in-hand. Our mental fortitude contributes to our ability to withstand prolonged physical effort. Given your injury is currently physically limiting, it is a great time to focus on the mindset portion of endurance!
As a marathon-runner myself, I can ensure you that mental strength plays an incredibly large role in my training regimen. Utilizing mantras and performance-enhancing thoughts have been proven to be effective in several studies such as these: Motivational Self-Talk Intervention on Endurance Performance & Talking Yourself Out of Exhaustion: The Effects of Self-Talk on Endurance Performance.
If your injury is allowing for running still, it may be a good time to try out some positive self-talk and mantras that will improve your performance. Meditation and yoga can also contribute to mental endurance.
In regard to your aerobic capacity (assuming this may have been more top of mind in your question given the HIIT/workout reference) walking, if still injury approved, is also a great way to keep cardiovascular health and endurance levels strong.
You won’t be reaching VO2 max or overexerting yourself as you would in a HIIT, but you can practice LISS (low intensity steady state cardio) to keep endurance up. LISS is the opposite of HIIT; the goal is to keep your heart rate (HR) around 50-65% of your maximum HR for a longer term, opposed to HIIT where you will be aiming for 80-95% of your maximum HR for a short period.
LISS has actually even been proven as an effective way to train for endurance events because exercising at a lower intensity for a long period of time puts less stress on your heart and lungs than a more-intense workout.
Your fitness routine may be impacted by an injury, but you can still maintain both your mental and physical endurance levels. Utilizing mantras and positive self-talk could lead to increased performance in activities you are typically less accustomed to in your training. LISS cardio will also be a great way to maintain (and even potentially elevate) your physical endurance levels.
"Easier" exercises from a HR exertion level perspective, can actually contribute to endurance.
Examples to try:
My fav LISS: 45 min treadmill walk at 3.5mph on incline of 15
My fav mantra running: "Easy. Strong. This is easy. I am strong."