The Finnish military had gone through certain major changes since the offensive phase of the Continuation War in 1941. Largely because of the economy (in a state of crisis) considerable part of the Finnish army had actually been demobilized in 1942. This was aided by the change of the war into the trench warfare phase of the Continuation War, during which both the Finns and the Soviets build opposing trench lines somewhat reminiscent of the Great War. While the Soviets launched some offensives (in 1942) the trench warfare phase consisted mostly of patrol raids and artillery duels with snipers being probably the biggest danger.
The reduction in manpower forced the Finnish military to try to adapt to the new situation with the organizational changes. There were several attempts of this between 1941-44. One was an attempt to change the organization of the 3 regiment (3 battalions each) + 1 detached battalion style divisions of 1941 into less unwieldy brigades but this wasn’t really followed through. However, what the Finnish military leadership eventually came up with was the reduction of the number of the regiment in each division by 1. This meant that was Finnish divisions typically had two regiments deployed at the front line that divisions no longer had any local reserves left, beyond the extra battalion.
Additionally, the long period the trench warfare had dulled much of the edge of the Finnish troops. And plenty of effort was placed on making the dugouts better instead of actually improving the defenses. The annual “best dugout” competitions being quite clear evidence of this. The poor state of the economy, and the need of horses (main means of transportation, apart from the trains), concrete, and men also meant that Finnish fortification efforts were largely incomplete by the summer of 1944. Also, as was typical around 10 000 men even from the front-line units had to be released for crop planting season in the Spring of 1944.
Furthermore, while the Finnish leadership was acutely aware of the military importance of the Karelian Isthmus the Finnish troops were not concentrated to the Karelian Isthmus but were spread out much wider, especially along the Svir river line as it was perceived that this area could be used as a bargaining chip when the eventual negotiations would begin.
On the flip side, while the Finnish manpower had been decreased this was something that could still be remobilized. Also, the equipment of the Finnish military had greatly improved from that of the 1941. Artillery, armor, anti-tank capabilities were far greater than what they had been in 1941. However, while these were improvements over what the Finns had used in 1941, they were not exactly the most modern equipment.
All of the above meant that the Finnish front-line forces on the Karelian isthmus (2nd, 10th, and 15th Infantry Divisions and the 19th Infantry Brigade) were very far from being the hardened in combat or at being in full strength. The Finns had just around 45 000 of infantry in the front lines in the Karelian Isthmus. And they were facing the Soviet 21st and 23rd Armies - consisting of 24 rifle divisions, 2 artillery divisions, 5 artillery brigades, 8 assault gun brigades, 5 armored brigades and 17 tank battalions, altogether around 260 000 men and over 600 tanks or assault guns.