1) It's a medical condition which, with proper management, leaves patients both physically and mentally sound. Patient (aged 25-35) needs to be fit enough for athletic activity.
2) The condition requires routine management, like dietary restrictions, taking drugs, or physical therapy. Without management, the patient would be sick or dead.
3) Knowledge/technology to treat/manage the condition must be available to middle class people in England in the early 20th century, before WWI.
4) The cause of the condition should be either genetic, unknown, an infection, or an injury (not a war injury). Starvation/malnutrition doesn't fit: the idea is that the character has access to his treatment.
Diabetes would be ideal, but the time period doesn't fit. AFAICT, without modern treatment, a diabetic person wouldn't live very long, and wouldn't be very healthy. But if anyone has good reasons to believe that's not true, I'd be interested.
My purpose for this condition is to have a character who is familiar with some kind of special routine of bodily upkeep, who is necessarily more conscientious about health than an average young man. A character who would be more than usually prepared for chronic illness. I think it could be possible to get this effect by having the character care for another person or animal(s) who isn't necessarily physically fit, and with that tweak I could have a lot more options. But ideally I'd like the character to have firsthand experience of the consequences (pain, incapacitation, etc) of failing to manage the condition.
Searches done: "diabetes history" -> learned that discovery of insulin treatment happened too late; "deficiency condition" -> learned that the most common deficiency conditions seem to be caused by malnutrition.