Maintaing/indreasing stamina... Please Help!

I’ve always relied on roadwork for my stamina. As a fighter i do roadwork pretty much every day when it comes to fighting and after reading the stuff on tmag about 400m sprints and HIIT (THANK YOU!!)
I noticed a huge increase in stamina. Anyway, 3 weeks now my knee has been playing up. It hurts if I try to run or even bike and I need to keep my stamina sharp so what excercises can I do to keep my stamina high?

I find skipping is ok for my knee (wierd but mabye it’s because im not bedning my leg as much as with running or cycling)

Will HIIT prinicples work with skipping? how does it compare to running for stamina?

Also what about swimming? What other excercises can I do? Running was great cos it also kept the fat off

thanks for any help you guys can give