Sorry to hear of your troubles. As you point out: In hindsight it is easy to see where and why the wheels came off. However it is often really hard to see those signs when you are in living it day to day. This is where a detailed training log can really save you. We learn best from our mistakes so this has not been a total loss.
As for starting over: The biggest risk you run is that you are not fully out of the over training hole yet. You must evaluate whether it is even advisable to start back to a structured plan at all. Do this by going out for a short (30min) easy run and see how you feel. If you still feel flat then you need more time to recover. If you feel good after and then still feel good the next day then try again with an easy run for 35 min. Do this for a few days. If easy runs wipe you out or if you legs still feel like lead then you need to rest. If you handle these fine then you are ready to ease back into training.
The component of your physiology that has taken the biggest beating from the long lay off is your basic aerobic capacity. So you need begin with several months to re-establishing you previous AeT pace and HR through longer Z1-2 runs and hikes. I’d not bother setting an ambitious hourly goal like 500 hours which is larger than the past when coming off over training and illness. You may very well find that just repeating a 425 hour year and doing it well will provide a boost to your fitness. Is it possible that you were being overly ambitious with those previous hour targets? If so training within your capacity might surprise you.
It is much easier to start low and bump hours up later than it is to start too aggressively and find your wheels coming loose again after 6 weeks