Coming back from injury - what's your story?

I've been feeling rather glum of late after realising I haven't ridden a mountain bike in nearly three years due to a rather complicated knee injury. I'm walking with a stick now but due to undergo surgery over the next two months, but I've been warned to manage my expectations - certainly no return to twisting sports like rugby, but mountain biking might be do-able especially with flat pedals.
Just looking to hear any stories of recovery from people who've been off the bike long-term.
By way of background I'm now late 30s, I used to be a good standard (raced Gorricks for uni; plenty of trips to the Alps and Spain), but I'd settle now just to be able to put in a 1 and a half loop on my local trails and get back into nature.