So Ki in this sense is physical? It can be conditioned in terms of thickness etc? Just asking.
In the sense that ki is related to connective tissue in the body, yes, while from a martial perspective it's a physical manifestation of intention coupled with physical action. It can be conditioned over time and skill can be built in better coordinating it's use for beneficial effects and application. From a body cultivation perspective it's an exercise in conditioning and coordination purity with some skill-building towards optimal use being by products of the transformative effect.
Basically, there's strength (alone from an IS perspective), there's strength + skill (alone from an IS perspective), there's strength + skill + coordination (alone from an IS perspective), then there's strength + skill + coordination + application from a martial arts perspective --> the latter being where your IS practice is embedded into how you've trained to express your practice of a martial system (that may also have sport or sparring or matches of some kind with varying degrees of competition to measure different aspects of your training).
That's at least how I understand it from a "classical" view spanning Chinese and Japanese martial system perspectives. Different practices and methods of cultivation may have aspects of the full complement of internal strength skills (my baseline is as defined in 6-harmony neijia arts such as Chen taijiquan) as probably best defscribed at MikeSigman.blogspot.com
Even today, some of the discussions and efforts at investing more recognized internal methods back into aikido (I know, I know, some say it was never gone, I appreciate the perspective even if I don't agree) are often talking about partial implementations, so to speak - where "ki" may be used to describe only the intention-force-management piece. Or the aiki taiso may be re-looked at for "ki building" as in how they work to better manage balance and build internal connection.
When people are having discussions about "ki" in aikido - I tend to look at it from the perspective of whether it's being measured against the "classical" view (as a set of strength, skill and body movement principles) or if it's more a philosophy or belief system. Understanding that even in the classical view, that a philosophy and belief system were eventually also layered on top (which I think is why the waters get muddied between mechanics and philosophy), I'm more interested in the concrete body mechanics around their expression. So even new methods of describing things, to me, should at least adhere or relate to some of the older methods at the very basic principles - even if the scientific understanding of what's going on may have become more advanced or sophisticated with more recent revelations of biology, chemistry, physics, etc.