Since the Romans had colonised England and built fortifications throughout, why did the Vikings not come into contact with British legions?
Did the population diminish so drastically that there just want enough people to continue the ring military traditions that would have survived in Britain after the fall?
While there will be always more to be said, you can check the following posts by /u/BRIStoneman on the relevant topic, also touching the post-Roman (Anglo-Saxon) military system at first:
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Neither do I think the Danes met only with "such little resistance" in England in the late 9th century, as I explained before from a bit different point of view in: Early Viking attacks on England failed as often as they succeeded. But after a few decades, they seemed to be far more effective. Did viking warriors improve or did viking leaders develop better tactics? Why did the Vikings become deadlier over time?