Hi chris67bug
MRI results like this are common. In the general population without back pain it is quite common to find disc changes like this. There is also a poor correlation between the presence of such changes and back pain. So just having the presence of degenerative disc disease in L4/L5. with a posterior annular tear in the same disc may or may not be responsible for the pain you have. Seeing this on imaging is not proof of cause of pain.
It would be impossible to say whether this is a result of a previous event or not. However these changes are VERY common in the general population so this makes it even harder to tell "cause and effect".
It is also true that there is very poor correlation between the severity of disc changes changes and severity of pain. You can have a lot of persistant and recurrent pain and the pain can be quite intense yet there be very few changes on imaging and vice versa.
There is no evidence that I know of that previous weight training would predispose you to back pain or degenerative changes. That is not to say that all weight training techniques are safe or that the more extreme end of training (eg professional weight lifting) there may not be some increased risk of changes.
In contrast sedentary lifestyle may not be good for back pain and there is some evidence that "getting fit" in the presence of chronic low back pain is actually helpful.
It is quite likely that your fears are unfounded. Participating in normal physical activity may actually do such tissues more good than harm. In fact the fear of injuring oneself (sometimes called "fear avoidance beliefs") is actually more of a problem when it comes to back pain. People who develop back pain and who harbour these beliefs and fears often appear to get into a downward spiral of avoiding activity and getting more disabled.I've always been an active person, I did weight training for years also which may not have helped my back much either. I am now worried about doing anything physical in case I hurt my back more and to be honest rather frightened for what the future holds for me.
The general wisdom nowadays is to get fit and keep fit. this is important for managing/preventing other health issues that in the end are far more salient to your health than back pain (heart disease, cancer, diabetes, obesity etc - the big killers) . However if you are worried about proceeding with this (for example "how do I manage physical activity if it is making me more sore?"), want some more specific advice then I would go and get help from a physio who is experienced at dealing with chronic back pain and who can work with you as you resume activity. Based on an assessment of your and your back they can advise on specifc exercises and on general issues with resuming a balanced fitness programme.
If you find your thoughts and feelings are really getting in the way of what you want to do then "cognitive-behavioural therapy" - a technique developed by psychologists for talking your way out of these thoughts - has been shown to help people with chronic low back pain. Please note I am not suggesting your pain "is in your head". But we do find this link between unhelpful thoughts and being more disabled by back pain