Given your training volume and experience training, I would personally consider 22 weeks to be plenty of time. Think of many of the top ultra runners who spend their winters skiing nowadays and who then transition quite rapidly into running season.
Recovery is quite individual and of course depends amongst other things on your training going into the event (that is, the 82 km race) and how hard you choose to run that prep race. Before A races, I have often done a B race maybe 8 weeks out; this longer spacing because I still would run it as a proper race, only that I was not yet in peak form for the A race. Admittedly, if I enter a race, I run to do as well as I can (only once I have entered a race purely as training for another race, which was mentally tough to accept), so I need some recovery time and then still a few more weeks training for the A race.
But you know best your recovery capacities, and presumably if you take it as a long training run and you go into it having a good training base, 4 weeks can be enough.
rich_b