Recommend? Right, all this is strictly IMO.
First, healing must be complete (or a long way down the line) before training that involves the knee(s) should start. This means no swelling or day-to-day pain, knee(s) pretty much out of mind, normal life continuing. If healing is not complete, stretching only, no training (this is what PT is for). Other training such as upper-body strength is unaffected, of course.
For endurance training I reckon walking and cycling (I've done both) and swimming (by reputation only, I hate it) are good.
So, start by walking a bit*. When the knee doesn't react over the next 24 hours, walk 10% more the next day. If you do get a reaction - or think you might be about to - back off (ice, elevate if needed) for one or two days, until back to normal. Then walk the distance that does not cause reaction and only increase the distance once every 10 days or so. Walk every day. Keep going until your knee isn't the limiting factor. Once you're going as far as you want, go faster.
Exactly the same principle on the bike, except IMO it's kinder to the knee(s). You can get your heart rate much higher and do a real workout. This really will improve cardiovascular fitness (endurance).
*a friend of mine, multiple world champion cyclist, 2 1/2 hour marathon runner and professional coach was once asked 'How do I start running training?' and replied 'Open the door and f*** off.'