Rick Steves Athens & the Heart of Greece tour - How physically fit do we need to be?

I think you’ll be fine. My husband was about your age when we did it and had knee issues, but other than not climbing up to the top of Monemvasia and Nafplio, he was fine and loved the tour. Both of those climbs were optional afternoon activites. I was anxious about the climb to the Acropolis, but the guide took it in stages, so there were no issues there and we did pay attention to our footing so we didn’t slip. It was one of our favorite trips.

I was 72 and my wife 70 when we did it in 2022. There are stairs and cobblestones. If needed your guide will arrange a taxi. They will help you on and off the boat to Hydra. As long as you are not in a wheelchair or using a walker you will be fine. Yes you can!

It can definitely be physically taxing if you opt to do the hikes that are optional, but they are just that - optional. My boyfriend and I are 39, but we had tour members in their 50s-70s who did the optional hikes up to the top of Monemvasia and Palamidi Fortress and had a blast. The parts of the tour that have a lot of uphill walking, like the Acropolis and Delphi, do take breaks throughout so it's not a complete slog. I strongly recommend bringing shoes that have good grip, due to all of the marble and slippery rocks you'll encounter. Wear good, grippy shoes, pace yourself, and hydrate, and you'll be fine :) This tour is a lot of fun, even if you decide to skip the tougher hikes.

Absolutely you should go on this tour.
We took this tour in October 2023 and we are seniors, as were many of the other participants. You will do a lot of walking, but most of it is flat. If you are fit, then you will have no problems. If not, You might want to practice walking before the tour to get in shape.
The strenuous part in Athens will be the climb up to the Acropolis, but the guide takes it slow and stops to talk along the way. So it is doable and well worth the effort.
Delphi is relatively flat from the hotel to the museum, (a slight downhill, so a slight uphill on the way back) but from the museum you will have to walk uphill to visit the Temple of Apollo. You don't have to visit the Ancient Theatre of Delphi which is a steep uphill walk from the Temple of Apollo.
the hotel in Kardamyli is up the road on the hill, about 100 feet from the flat main road, but it's a doable short walk.
You can skip the hike up to the top of Monemvasia, as most of our group did. But you will walk through the old town of Monemvasia and maybe out to the lighthouse and back. Easy enough. and once you are back in the new town of Gefira, you will have to walk uphill back to the hotel, if you stay at the Monemvasia Panorama Hotel. I remember that uphill walk as it was a hot day. So bring or buy water. You will have to walk down and up that hill to get to the restaurants at port side.
The hotel in Nafplio is at the top of the hill, so it's an easy walk down, but an uphill walk back. The hotel took care of our bags, so we didn't need to haul them uphill and they were transported to our bus when we left. Some young hardy folks hiked to the top of Palamidi Castle, but most people on my tour did not attempt that hike, (we didn't) and there's no need too. There was a short down hill from the hotel (which is at the top) to the rocky beach on the back side of the hill, so it will be a walk up back to the hotel. Just take it slow.
At the hotel we stayed at, we had to walk up outside stairs from our room to get to the breakfast area each morning. Works up an appetite .
Mystras was a steep hike up to the top to a convent. Several in our group decided not to do the hike and waited below under a shady tree.
Mycenae is a gentle uphill walk to the Lion's Gate, but beyond that is an uphill hike. The walk to the Tomb of Agamemnon is an easy one.
The walk to Epidavros was an easy one.
On Hydra, the walk from the pier to the hotel is a slight gentle slope. Almost un-noticeable, unless you are out of shape. We paid a porter to bring our luggage from the boat to the hotel. Your tour guide will facilitate this.
So you will walk a slight grade down from the hotel to the port and the restaurants, but then you walk back up. But it's all doable. You can stay in the town area to sightsee but the walk to several beaches is on a hilly up and down path.
Just inform you tour guide of your concerns. One older couple was placed in the easy access hotel rooms to minimize their up and down walking.
So yeah, I guess strenuous is a good way to describe this tour. It's not a walk in the park, but if you are fit, you can do it.