HI MichaelJohn
Not a great feeling loosing that fitness eh! How long since the injury? And was it a partial rupture? The physiotherapy management for PCL is a bit controversial as there has been surprisingly little quality research in this area. Firstly I wouldn’t do anything that stresses the knee without getting a go ahead from the surgeon.

In many protocols cycling is can be introduced two weeks post-injury, but it must be pain free and you need to have the range of movement to do it. I would get advice on this

One completely safe thing to do is to get an aerobic workout without exercising your legs. So if you enjoy swimming and can get to a pool you have the perfect solution. Freestyle, and backstroke would be fine but make it all an arm activity – use a float between your legs to provide buoyancy. Aqua-jogging and introducing kicking might be ok later in the piece but get advice on this.

If you have access to a gym you can do upper arm weights, and an arm ergometer if they have one is great for aerobic. Just avoid anything involving weight bearing through the legs – eg know squats etc. You can also work the uninjured leg - why not get a physio who works in a gym to devise a programme.

You should be able to work on your quadriceps and hamstrings muscle if you are more than 2 weeks post injury. In fact the stronger the knee muscles pre-surgery the shorter and better the rehab. But it needs to be controlled. - again get advice from a physio who has assessed your knee.

Get some help, Michael. No need to turn into a potato! Even getting some dietary advice might be sensible thing so that you don’t start packing it on when it is hard to burn that fat off. It is much harder to get rid of it than prevent putting it on.