I have done this trip, but not the pre- or post-trips, so I cannot speak to those extensions. I don't recall any time that we hiked very far at a single time. There was one day we took advantage of a short forest walk, but hardly a hike. Probably the longest "hike" was the foot tour of Buenos Aires. There are some challenges to climbs when exploring the different landings from the ship, but one needs to assess each situation and decide individually to go or not. Some of us didn't trust the footing, but some did and they chose to climb over the snow and ice-covered areas. The rest of us stayed near the shore. (And after you have seen the first 400,000 penguins, well --- what more can one say?) Do try the lava-heated waters; it is a hoot. Some chose not to do any landings or walks. They enjoyed watching from the ship.
You might encounter thin ice under the snow and may step into holes, especially near the beaches, so be prepared to ask for help to get out (of course, that is part of the tales to tell when you get home). Also, be prepared for penguin poop -- you will get it on you and there is NO WHERE to wipe it. The dirt you think is on the ground is poop!! You can rinse your hands in the cold, cold sea, and the guys on the ship will scrub the poop off your pants and jacket and you have a boot bath to walk through both going and coming to/from the landings. Again, part of the experience.
Go! Have a fabulous time. Say "HI!" to the folks at the trading post. They are a hardy lot.