Well rawlings, for one thing horses heal much better than humans, for another thing permanent injuries are obvioulsy possible in any species but they are relatively rare. I can actively argue that just because your horse is injured, that doesn't mean the injury should be permanent :
Maybe the running thing can be blamed on age rather than the accident? I wouldn't know though since I'm not you lol but I used to be slightly fitter right after the initial training phase during my army days, towards the end I didn't run as fast etc. but didn't have any accident or anything.. Like I said I wouldn't know what happened with your knee though so won't comment too much :
Gratz on getting your wings (although it was some time ago) btw
Personally I haven't really heard of a horse who has permanent injury is is thus slower than before after healing, but most people would put the horse down unfortunately.. The 'lame' horses which I did encounter, were relatively minor injuries, the worsed being a horse falling into a drain and having his legs deeply cut. (needless to say he was lame for quite a while after that) Legs are primary survival tool for horses, whereas humans can live without them. Its something like natures defence mechanism, eg. in the wild a horse who has a screwed up leg even after healing would not live...
Any case, point B is that realistically speaking, after healing the horse would not
immedeately regain its previous status, since he did not excersise, lost muscle, gained a bit of weight and is somewhat 'stiff', so the horse would need a bit of training / schooling before becoming just as maneurable etc. This, in my oppinion would be a welcome addition, but it once again brings us to training horses. I put up my suggestions here http://forums.taleworlds.com/viewtopic.php?t=4693 for new members who haven't seen it yet.