Agreed. If your objective is to burn fat tissue there are optimum HR ranges you should maintain for that time. Higher HR's go anaerobic which is common in HIIT.
Irrespective of the machine you use the HR range and time on those ranges are what matters.
I've been using Polar HRM's for 15 years. Easily worth the investment to wash the guesstimation out of your training.
The link below has some information on target HR zones and a theoretical MHR calculator.
http://www.polarusa.com/targetzone/default.asp
I know the direct question was about Cardiovascular conditioning, but my curiosity is getting the best of me. When did you have surgery? How many weeks of PhysTher have you completed?
Did your PTX look like this? http://www.sportsdoc.umn.edu/Clinical_Folder/Rehab_Protocols/postop%20%20acl%20recon.html
As for the boredom? Maybe techno music will do it for you, personally I'de have a seizure from all the gay. Understand that passing is mandatory to live your military dream.
PM me if you want help working up a plan that doesnt interfere with your studies or put that ACL at risk.
Good luck...