I did this trip in February 2018 and I was in a group of 9 like yourself. We travelled on a different Hurtigruten ship to you but I'm sure it will be at least the equal of the Midnatsol. The crew were absolutely fantastic to make this truly a trip of lifetime!
The tips I would give are wear layers! You may not believe it but when we walked to the top of some of the snow covered hills (not as strenuous as it sounds!) you needed to unzip a layer or two! The ship will provide insulated boots for use in Antarctica and it is mandatory to wear them for any landing. I would recommend a good pair of walking socks.
Also don't forget your seasickness tablets. You may well not need them, I didn't but best be prepared.
Also you have get a medical declaration completed by your GP about 6 weeks prior to going and you must take it with you when you board the ship.
Hope this is of some help.
I wish that I was coming with you!!!!!!!!