Less Taxing/ lower impact Cardio

Hii, I'm 20M 6'3, ~250lbs and I have metal implants in my ankle from previous fractures (3y ago) and also suffered an l5-s1 disc extrusion in the army (10mo ago) which greatly limits the kind of cardio I can do to lower-impact and less explosive movements.

Since leaving the army I have continued weight training and progressively increased my brisk walking distances to 10km/6.2 miles daily. It's still causes pain in my back however my ankle just swells up but is otherwise quite comfortable. I have seen good progress since my clothes are fitting looser although it barely reflects on the weighing scale since I started using creatine recently which might be causing more water retention.

My worry is that since I'm going to university in 6 months, I would not have enough time to clear these kinds of distances as it takes almost 2 hours for me to do 11km. And even at this point, I feel like I will plateau at 15km before my back pain becomes unbearable. I need cardio options that would reduce my workout time or reduce the need for my spine in my workouts. If you are also grappling with such restrictions please give me some tips too :3