You have to find out what you can do. Eighteen months has probably wiped out most of your previous gains.
The usual gentle ease back after injury is to pick a C25k week that you think you might manage to do ( to be frank in your case it may be back to week1). Then try one run. If it is obviously easy, move on a week etc. Obviously if you try to step in too deep, you may have to go back down the weeks to find your base level.
Working through from W1 may be your only option and I would say from experience, having been injured a few times, don't rush it. Just because you used to be able to do it, does not mean you still can and slow and steady is as important coming back from injury as it is in the initial steps.
Sounds horrible and hope you continue recovering. It might be worth getting some advice from a physiotherapist on how best to start, you are going to need to ensure what you are doing is safe. Assuming you are cleared to start, I would just try week 1 run 1, slowly, and see how it goes.
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