Guru, do you live in HK? I see that you also posted about the flight grading. If you do live in HK then a visit to the HKAOA website may not be of too much interest at this stage. I thought for a moment you were thinking of moving here.
To answer your specific's; As an S/O, after the first 6 months back at CX, (after Adelaide) you should be checked out to the line. As an ultra long haul pilot you will typically do a 3,4,5 or 6 day pattern followed by at least 5 days off then into another pattern. Hours build quickly so in a month you MAY get up to 14 days or more off depending on trips. 5 or 6 days off is plenty of time to visit Thailand or the phillipines etc on days off. I'm sure you will be busy in your spare time.
The younger you are the easier the ULH flying is, so it's hard to say how knackered you will be. In general the day after I return from an ULH sector it is a right off. I guess after 20 years of ULH flying you might be a bit shagged. Much depends on what you get up to while you are away and how your body is able to cope. It differs for everyone.
It's hard to detatch yourself completly from work. Many of us would like to but flying is more than just turning up at the "office". You will find you have a great deal to learn in the early years and quite a lot of home work is required if you want to do the job well. It's also hard to detach yourself from work if the company is shafting your lifestyle with disruptive rostering or ever decreasing benefits.
You will have an annual medical up to age 40 then a 6 monthly medical until retirement. Pays to keep fit. You can do anything you want to do sport / hobby wise but injuries in some high danger activities may cause your company insurances to be invalid. The list of exclusions is extensive now (far more than 10 years ago) but there is no rule that you can't do something. Just don't get hurt!
Hope that helps. It's not a bad life but you are a tall poppy in our company and many people love to try to chop you down. You will enjoy the people you fly with and if we could just get better management the company could be great again. Best of luck.