Well that is pretty sad - sorry to hear that. I had to come home early last month as my old knee trauma flared badly again - can't do it any more so that was my last walking Camino (will return to do first aid with my car in future).
All depends on how mobile you are really - rest usually means rest! but ....
Does depend on the knee injury but often cycling causes no pain - as the knee isn't swivelling side to side and isn't having weight pushed through it - if it is that sort of injury then you could buy the cheapest bike there is and continue by bike?
But if "stuck" in Burgos .. able to walk a little? Are you squeamish? How about doing first aid for pilgrims? You could visit the biggest refugio every afternoon until evening then join in a pilgrim meal (for the social side of things), then be there first thing to help before they leave? Become the Burgos Camino Samaritan? If that interests you message me and I will send you all the info you would need, from how to do blisters to what to buy.
Go to the largest refugio, explain, ask them if they would like you to help on the welcoming desk for all the incoming English speakers every day?
Or ... well .. what skills do you have? Massage? - that would do well, just pick the biggest refugio, explain, and go from there.
Can you cook? If there is a refugio with a good kitchen .. could you set yourself up to do a pilgrim meal every evening? Take in five euros from each pilgrim who wants to join in, cook a fun meal every night? As there are new pilgrims each day you could do the same meal each day ... ?
Go to Santiago ... make a sign saying "information for English speakers" or similar, put it on a length of wood, buy a folding chair, tie it to the back of that, and set yourself up in the cathedral square? Find out everything there is to know that pilgrims want/need to know .. how to get a compostela, where to stay, compostela laminator shops, souvenir shops, toilets, best cafe at best price, where Ivar's luggage store is, how to get to the airport, nearest pharmacy/doctor surgery, etc, etc .. get free maps from the tourist office ... be a fountain of knowledge and help all pilgrims that speak English?
eerrmmmm .. all I can think of ...