Hiking in Meteora

I think reasonable fitness is sufficient given the many bus tours that visit, but I also think you would really need two nights so you can hike over one full day (and recuperate!). I'm in my mid forties and that is what I am planning. Though transport will likely determine how little time you could accomplish it in.
The Bradt Guide to Northern Greece gives the time for walking between the six monasteries as about three hours, but then you also have to walk up into them in some cases.
There are also additional hikes, but I am still researching that--I would like to walk through some of the rock formations and escape the crowds a bit--and I don't feel compelled to see inside every monastery.
I am coming from Thessaloniki with a car. Allow me to suggest something you may be able to fit in from Athens with less time-- Dimitsana in the Peloponnese. It's closer to Athens and there is a lovely hike to two monasteries there.

Valadelphia thanks so much for your reply. We will be in Nafplio before Athens and your suggestion of Dimitsana looks promising! I will look further into exploring this town.

Yes definitely do so--especially if you think you'll be heading back the next year as we did! It is hard to fit everything in because there is just so much and distances can be vast. We stayed in Nafplio and Kardamyli, and then stopped near Dimitsana on the way back to Athens (it's a popular weekend getaway for Athenians). The towns are picturesque, there are many good hotels, and the hiking was really beautiful. One of the monasteries is really dramatic like the ones at Meteora and the other is modern but historically significant (and the hike to it was gorgeous). There is a super cold stream also--that would be perfect in July.
Of course Meteora would be worth the travel, but if you do want to maximize your time I recommend it heartily. enjoy!

You have five months until you go. I would suggest starting a walking and stair master program to prepare. We are in our early 60s and I will tell you it makes a big difference in how much you will enjoy your visit. We went to Greece with our young adult children a couple years ago and they of course did nothing to prepare. But because we did prepare we were able to keep the same pace as them.

We were in Meteora last July. It is not hot as it is up in the mountains. In fact, the peaks in the distance still have snow on them.

I would not do Meteora as a day trip. 4.5 hours each way. If you overnight, I recommend Doupiani House. https://www.doupianihouse.gr/

For hiking, you can do it unguided, but if you want to go on a hiking tour, I can't recommend Visit Meteora highly enough. https://www.visitmeteora.travel/

You do not mention where you are from--are you used to hot weather or does it make you feel drained? Are you sensitive to elevation changes? The heat in Greece is hot and dry and you will also have some elevation change. While it will be cooler in the mountains, you will still have direct sun on you during the day. With that in mind, if you are reasonably fit, your best bet is to get early starts to your hiking days and with some water and snacks in tow you should be fine. I would also recommend that you stay one night to do some hiking over parts of two days if that is important to your experience. A day trip would be better if you just go with a bus tour as there will not be a lot of extra time to wander and explore. Either way, you are in for a stunning experience.

Thank you for all your replies. I have decided to add an overnight stay to enjoy the Meteora sights and will make sure I am fit enough to hike the area! Darrenbois the hotel you recommended looks stunning! I'm definitely going to try and book it. Are there nice restaurants in town that you would recommend?

You will find a few restaurants in Kastraki (near Doupiani House) and lots in Kalambaka (main town), but nothing we found special.