Hi brbo
Your story is not too dissimilar to my own. I have just come back from the bitumen & onto the dirt on a R1200GSA about 6 months back, after 18yrs.
I too contemplated the training, but have found that getting out there & riding, it is all coming back in waves & no problems so far.
Although I found the weight & horsepower a bit much to start with, I have now got it comfortably power sliding & climbing terrain that may be a bike this size shouldn't  Only one  so far 
Find someone to ride with and go for it. ENJOY your new bike 
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My memory is becoming that good, I should be able to organise my own surprise party soon
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