I don't think you even need to have that many marches. The problem with marches is that they're usually boring as shit, so having a march for the sake of a march, is bound to lead to a low morale on the part of the recruits. If instead, you're marching to an end destination where you actually get to do something, then the march itself doesn't feel as frustrating. At the end of the day, people need to feel like what they're doing has a function or a purpose of some sort.
I think the low level of fitness in the military is explained largely by the same reasons as to why people overall have a low level of fitness, these days. Too many machines are being used to compensate for manpower, which leads to men having a lot less to do, physically. Instead of lifting boxes of equipment, hauling all of their gear alongside them at all times, everything is being delivered, built and sorted out by vehicles or machines that assist in such work. Which might be fine in a military situation, but not when you're trying to build people to withstand those military situations.
The one good part about the local Finnish army is that, atleast in parts of the region, the infrastructure is borderline ancient and you don't really have anything comfortable outside of old shacks where people get to sleep the night. But it's inevitable that technology will replace much of the physical work required in the army, and that comfort levels will increase, so the military needs to start looking into some type of a conditioning program to whip recruits in shape, instead of relying on people picking themselves up for it.
The military just isn't "tough" enough these days, to force people into regimenting themselves in order to keep up. Previously, if you were out of shape in the army, you only made it harder on yourself. Today, you can be in a pretty bad condition, and still hang in there relatively comfortably. So there's less of an incentive to start looking into ways of self-improvement, in that regard. It has to be the army itself, which takes the initiative to improve people's condition.