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+40 and you'll be un-neededly fresh. You'll come back without the big overload you desire. You want to be on your knees at the end as that's overload.
I started transcontinental at +30 and that was perfect. Don't become untrained just to ride hard, just freshen up and then take the first few days at caution then ride yourself into the ground as he week ends, then when you get back take some time off and some way days and let your body recover. Use your time off to recover after riding hard not to become untrained before.
Just my opinion.
Better to be over rested than over trained