Steroid injection - advice please

Let me state clearly that I am not a medical person, I am an engineer. Steroid injections can reduce inflammation, swelling and I suppose hence also reduce pain and discomfort and stiffness. I believe that one of the worst things that you can then do is to over-exercise, thinking that the magical steroid injection has fixed everything. The benefits of a steroid injection can wear off, they are not a cure. I am now in my late 60s. At age 21 while at university I injured my knee playing rugby. At the time is was disgnosed as a "bad knee". At the end of the season we staged a 700 mile run for charity - it involved 15 of the club running in relay, 700 miles around the roads of Warwickshire over 3 or 4 days. After my first day where I had run about 20 miles in 2-mile legs, my knee was killing me. I went to the GP on the campus surgery, he was a strong supporter of the rugby club and often helped out with popping-back dislocated fingers and similar and similar on match days. He understood that I wanted to continure, so he gave me a steroid injection into my knee. I continued running for the next few days. Once I finished and the stroid wore off - I was in agony. I later discovered that I had damaged my knee severely. Now, with maturity and appreciation of my physical mortality and limitations (something I ignored at age 21) I understand that after a steroid injection then gentle excercise and gentle physiotherapy might be beneficial, but you should not simply resume your previous regimes. You can do further damage that only becomes obvious when the effects of the steroid wear off.