Hello, I wrote a longer answer here that looked at the track records of the Danes in conflict with the various Early Medieval English kingdoms.

The largest part in the success of the Danes, at least prior to the 860s, is that in the large, they don't try and fight the English, at least unless they have local superiority. Danish raiders struck principally at isolated targets - in particular religious communities - that were lightly or not defended at all. In the 840s, they were able to defeat the Mercian army before the West Saxons could support them, but, it's worth noting, that they were subsequently defeated by Æthelwulf's West Saxon army. As my linked post described, the Great Army that takes Northumbria and East Anglia in the 860s capitalised on a period of great civil upheaval, and even open civil war, in Northumbria, but had mixed results at best when combatting the prepared Mercians and West Saxons. Indeed, the record of battles in 871 shows that largely, Wessex defended itself well, at least until the degradation of manpower caused by the need to constantly defend against raiding weakened its army in the field.

The West Saxon and Mercian defence against the Vikings is far more effective following the burghal, naval and fyrd reforms of the 870s. It's worth noting that from the 900s, the Danes in England are in fact largely on the back foot, and during the reign of Eadgar, the English navy was notable in preventing a single hostile fleet in reaching English shores. Of course, the defining factor in this period is an increasingly centralised and strong bureaucratic government.