Here’s my current personal endurance training routine:
one night: 1000 push ups & 1000 double calf raises
next night: 1000 air squats & 1000 sit ups
repeat without skipping a night.
Going to start working on a pull up goal.
Never under estimate what you can do, nor think its too late to do something about it. Best wishes on your personal goals and to exceeding them. Also, buy enough sweat shirts if you do hi rep sit ups, I regularly wear a hole on the back of my sweatshirt-going through a sweat shirt each month