If pain doesn't set in until 30-40 miles (say, approx 2 hours) then I would guess it is not a big issue with fit. Assuming same stem & bars (not a good assumption) they should be approx 1" (24mm) higher on new bike and only 4mm further out. Given the range of stems/bars and posts it would be easy to hit the same position as on current ride, or create a slightly different position.. There are short/shallow handlebars that make the transition from tops to hoods to drops a shorter distance.
Not being comfortable on the hoods, just the tops, is a problem as the braking position is compromised.
Hand pain can come from many things: too much weight on hands, bad gloves, same hand position for extended periods, a nerve issue, etc.
30-40 miles of gravel (even on this fat tire bike with drops) would likely give most riders some aches. Especially if you are cranking pretty hard.
I often set-up my bikes for comfort in the drops which makes riding the tops or hoods an easy reach.
Good luck!
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Last edited by Wildwood; 11-29-19 at 12:14 PM.