What should I do about recurring knee pain and weakness in a 21 year old?

I’m in awe of your dad! Kudos to him for his commitment to keeping fit and healthy. If only the rest of our population could do that – we would certainly have much fewer people in hospital. In the absence of any other information leading up to his knee injury (ie did he suffer any knee trauma, suffer a fall while running, hear a pop and twist his knee etc), his injury sound like it’s training volume-related, or an overuse injury – he’s clocked up quite a lot of miles, and it seems like even with his injury, he hasn’t actually held off with his training.

I am sorry to hear of your symptoms. Fainting as you have found out by now has many causes though the vast majority are benign. But, it appears that you have already had a thorough clinical examination and investigations as you list MRI and heart screening amongst them. However it would be useful to know whether the MRI was of the heart or brain and whether ECG, Echocardiography, Holter, exercise stress test and Tilt table test were part of the tests. Your family history is also important.