clearstream
(He, Him)
For three or four years I ran a civil war campaign set in 14th Century Damara. I found it very helpful to estimate the forces up front. That allowed me to organise them into flavourful groupings that the PCs later joined or interacted with. Such as a rangers contingent drawing inspiration of course from those met in The Two Towers. I agree with you that the simple tiers method proposed probably low-balls it for a kingdom on a war footing. In addition to creating veterans, a war would draw in mercenaries. IIRC I roughly doubled the number of cc-equiv NPCs during the war period. And I removed a large part of the population (dead or enslaved) so the end density would have been higher again. My standard unit was a mixed force based on a medieval spear.Honestly, I would try to not think about it too much, because the concept of HP and rapid recovery just don't work on a large scale. If orcs are rampaging the countryside, and half of the adults are pressed into service to fight back, then a significant number of them will see combat and gain combat experience. The ones who don't die will eventually reach the point where they are better represented by the veteran stat block than by the commoner stat block. Some of them may get there more quickly than others, but it doesn't really matter, because it ultimately comes down to the DMs interpretation as to which stat block to use (or whether they want to implement one or more intermediate or advanced stat blocks).
A streamlined approach could be to triple the numbers for a war footing. If we say that peace time Heliogabalus has 440 T1, 44 T2, and 4-5 T3 which chimes quite well with the various arcane groups, the paladin orders and so on that we know about. A good proportion of the 132 T2s will be Veterans and each one will have 10 footmen (Guards) or archers (Scouts). Damara is a small kingdom far away from anything so isn't going to see the sort of massed affair one might have in the South.
Of course every DM can and should have their own interpretation. I'm aiming to offer a starting point, a quick rule based on an oft-repeated DM guess of 1/100 Level 1s. Through using tiers, it remains vague enough to work for a wide range of campaigns. I will add something on war, and [MENTION=12731]CapnZapp[/MENTION]'s point about Hit Dice, to the OP.