Seconding this, I feel compelled to put my men through every encounter humanly possible if only because getting them to a point where they can shoot semi-straight or move at more than a brisk jog takes forever.
Even the slightly better than raw recruits you start with are frankly painful to use for the first two dozen missions, considering they're barely statistically on par with scrub alien non-combatants despite allegedly being hand-picked combat vets, and the gap only grows ever more obvious as the game progresses unless you force them to improve at every opportunity you can muster.
I've always thought that there being more a focus on your troops being outmatched due to technology rather than physical fitness makes more sense in context, considering all the in-game lore about their poor adaptation to terran atmosphere. Starting out, trained troops in their physical prime are being outmaneuvered and outperformed by the alien equivalent of office workers with asthma.
Perhaps putting less emphasis on increasing troops physical stats (currently being able to nearly double virtually all of them to the 100-ish range) by giving them higher default stats, but also a slightly lower maximum ceiling on improving those scores would work. Say, a troop who starts with 50 strength would only ever be able to increase it to 70 at most, meanwhile someone who started at 80 could easily max it, representing the difference in their current soldier conditioning versus the absolute best their body could ever allow for.