There are a TON of things people can do in place of running. HIIT and TABATA training provide great cardiovascular workouts. 

Cycling, rowing and swimming all work well also. 

I trained Strongman for several years and we would drag weighted sleds or suitcase carry at a quick pace with short rest intervals for cardio. 

Step and aerobics classes are also great. I took a step class and a power lifting class for electives during one semester of college and I was peaking pretty good by the end of the semester. 

Honestly, if you want absolute “peak” performance even running should be trained in intervals. Whether it’s a 5K or a Marathon. 

I’ve done all the above at different times in my life and never failed to do 5-7 mile hikes at high elevation with a 50lbs pack. 

I guess it all boils down to how you define peak performance. 

P.S. If you really want to be humbled try jumping rope without stopping for the same amount of time it takes you to finish a 5K. Every time you miss, add a minute. 😈