I believe soccer conditioning is best developed on the practice field (mostly playing soccer).
I am 11 years younger than you. I played soccer for 10 years in my youth and 10 years as an adult. I coached it for 14 years at the youth level. I quite playing after the second ACL tear.
My understanding is that you use the term conditioning to refer to endurance. The soccer book I referenced seems to use it to mean developing strength, power, speed, speed-endurance, endurance, strength-endurance, coordination, and flexibility. Ideally, the practice is economical in that it is developing technical, tactical, physiological, and psychological ability in the players at the same time. Sometimes, you do focus on a single area.
There are different needs in diffeent roles for example a keeper differs from a midfielder.
I agree that new player who practices for two weeks, and plays a game will then be at a better fitness level. Still, there is a big gap from this level to the level of a player who plays year round and maintains their endurance during the offseason.
Most cross country runners will beat most soccer players in a 5k race. It is aerobic and running in a forward direction for 17-18 minutes. The sports have different demands.
I do not know anyone that can get sub 18 min 5k in only 3 weeks of training unless they had a prior level of fitness or are very blessed genetically. So, the demands of cross country are not easily met as a short-term adaptation.