I have not taken that particular tour, but have been on 10 other RS tours. The walking tours are not forced marches! Even on the most “strenuous” days, it’s more like walking for a while, stopping to hear the guide talk, walk some more, take a break, walk again, have some free time to explore on your own (or sit in a coffee shop!), etc. I am 73 and don’t exercise all that much at home, but have not had any problems on these tours.
It’s up to you how much you participate on the tour - it’s your trip! You can skip almost anything as long as you inform the guide so he’s not worried about you. For instance, on a city walking tour, you could take a taxi back to the hotel if you get tired. Obviously when the group is changing locations and hotels, you have to go along, but even then, you could wait at a cafe for the group to return to the bus (you can’t actually wait ON the bus because this is the driver’s break time and he usually keeps the bus locked). I will say, however, that the guides and activities are so great, you won’t want to miss a thing!
I find that the days I walk the most are often days with a lot of free time where I pack in a lot of extra sightseeing. This is what I like about RS tours, a nice balance of organized activities and free time. You, of course, can do as much or as little as you want during free time.
As to luggage, a smaller suitcase is really best as you sometimes have to move it yourself several blocks to and from the hotel. This is the trade-off for staying right on the heart of older European cities and towns. As long as you can handle your own luggage, you can bring whatever you want, and I have seen some people on these tours with very large suitcases. However, I have traveled for as much as five weeks with a 21” rolling suitcase (I still check it on the airplane because I hate dragging it around the airport and lifting it over my head) and a large tote. Most hotels where you stay for two nights will do laundry for a price (I splurge on this often). You can also do sink laundry (I don’t, except for very small items) or use local laundromats (kind of fun in Europe). Your tour leader will give advice on this.