shown as being far superior to the English in fighting ability, tactics and basically everything in between

When the English faced the Danes in open battle, they tended to win. I wrote about the English track record in defeating the Danes in an earlier answer here. What makes the Danes so dangerous is not their fighting ability, but their manoeuvrability. Fast moving and widely dispersed Viking groups were able to raid across wide areas of England, overrunning small garrisons, sowing chaos and discord, and then fleeing before properly organised military forces could respond and force battle. In mass raids like that in 850, Danish forces were able to successfully isolate Brihtwulf's Mercian army before Æthelwulf of Wessex was able to combine forces with him. We can see how dangerous this makes the Danes when the Great Heathen Army attacks Wessex in the early 870s: in his Vita Ælfredi, Asser notes that Wessex is being 'bled white' of manpower by 'innumerable' small raiding forces ranging alongside the main Danish force. This forces the West Saxons to either continually counter-march against them until they hit exhaustion, or to continually split their forces to provide small local garrisons or risk desertion as men leave to defend their lands. Either way, this leaves two bad choices to West Saxon leaders, either face the main Danish force dangerously tired, or dangerously understrength. We can see this problem in action jn both Asser and the Anglo-Saxon Chronicle's account of the battles of 871. At the Battle of Wilton, Alfred's West Saxon army initially achieved a quick victory, routing the Danish centre despite being heavily outnumbered. Alfred's army was unable to pursue the fleeing Danes however for fear of being encircled by the still-intact Danish flanks, and instead chose to withdraw, allowing the Danish army to regroup and claim the field.

The change in England comes with Alfred's institution of the burh fortresses and their fyrd garrisons. I wrote an answer about what burhs were, how they were sited, developed and functioned here, and how the Fyrd was selected and recruited here. Essentially, burhs were fortified garrison sites at strategic transport loci which served to prevent Danish forces accessing the road and river routes which had granted them such deadly strategic mobility, and instead provided that tactical flexibility to standing English garrisons. The burhs and their fyrd garrisons were connected with each other and outlying settlements by a network of watch-sites and signal beacons. This allowed English forces to quickly identify and effectively surveil Danish forces, and marshal ready forces to intercept them far more readily and effectively than had previously been the case. It also freed up the main army to campaign more effectively against the Danes.