OK. Thanks for the extra info. I agree, overall fitness is not the issue. Anyway, as with the stock market, past performance is no guarantee for what happens in the future. The bigger issue also for me was amount of experience with hiking in and around the White Mountains. A person can only hike the region. That's the only way to learn how to read the terrain.
If you lost your way inside the Aradena gorge, and you also have some issues with vertigo, then the Ag Ioannis to Roumeli route may not be for you, especially if your experience has been limited to some gorge walks. This is not something I can decide for you, of course. But it might be better to (i) try something easier and more popular first to evaluate both yourself and what you have to deal with and (ii) always have backup plan to abort hike if it seems to be too much.
Photos might be of some help. Here is a Google overview of the Ag Ioannis - Roumeli route:
The trail from Ag Ioannis goes to the cliff at a place called Sellouda. I have marked the location of the Ag Pavlos chapel, and you see Ag Roumeli in the distance. Here is a view towards Roumeli from Sellouda:
Here is a view looking down at the start of the Kalderimi. It gives you an idea if it will cause vertigo issues:
Here is another view of the terrain, a photo taken more or less at the point marked with an 'X' on the map:
I don't recall there being paint markers on this section. You have to be able to see the correct trail, know where you are more or less going, and so on.
To be honest, if you had to ask if there was a trail between Marmara and Roumeli, then you probably don't yet know enough about the region to be able to take on too much. Certainly investing in a map will help you a lot. Anavasi publishes the best hiking maps for the Sfakia area.
The Marmara-Roumeli route is almost guaranteed to have quite a few other people on it, just in case you get disoriented. However, they usually start early from both ends (Roumeli and Loutro) between 7-8am. You may want to take that into account when you make your own plans (time of day when on the path).
I don't think that mild vertigo sufferers will have much issue with the Marmara-Roumeli coast route. I can post a few more photos to give you an idea what to expect, if you're interested/curious.