My family and I have done both Iceland and Norway. Of the two, I would pick Norway. Norway has to be among the top
ABD trips we have taken. Although, to be fair, Iceland was done during covid and some activities (like the river rafting) got cancelled and replaced with other events.
Most of the activities on both trips were done over flat ground or over small inclines. So as long as you are able to walk a ways (like from the bus parking lot to a beautiful waterfall), you should have no issues. Not everybody paddles during river rafting. People do what they feel capable of doing. [Inside tip: you wear a wet suit and life jacket during the river rafting, and at a slow part of the river people are allowed to jump in. Be forewarned -- the river comes from melted snow and it is COLD!!!]
In Norway more than Iceland there are several optional hikes which involve going up/down several hundred feet of elevation. Not everyone partakes in those and there is usually an alternative activity for those less inclined (pardon the pun). For example, in Iceland my wife, son, and I did the glacier hike while my 22 y/o daughter and others went with the other guide on the bus to the rocky coast to see a puffin colony. You do what you feel capable of doing, and one of the ABD guides always stays with the slowest adventurer on the hikes to ensure that no one gets left behind.