When to use the imperfect:
The choice between these is sometimes confusing for English speakers because all the forms in red (i.e. all except for caminó) are frequently translated as was walking. So what's the difference?
There are many cases where the simple and continuous imperfect tense forms are more or less interchangeable. In general:
The last point refers to verbs such as parecer ("to look, appear")1, faltar/sobrar ("to be lacking/surplus"). For example:
Both of these forms are commonly translated by was/were ...ing in English. They both refer to actions that are viewed as being "drawn out" over a period of time. But the preterite (i.e. estuvo caminando etc) also views the action as completed. So it tends to be used when the speaker wants to emphasise that an action "went on" for a period of time, but where they also mention a phrase such as durante X horas, toda la mañana, hasta ayer etc: that is, a phrase that delimits the action. Compare, for example:
In the first case, the imperfect (continuous) is used as the speaker doesn't actually state when the person stopped working, just that they were "in the middle of" working at that point. In the second case, the speaker actually delimits the period over which the person was working.
Note that the imperfect is generally used to describe a repeated action that occurred within a delimited time (e.g. "all night") but where overall the sequence of actions is not delimited. For example: estaba trabajando toda la noche would generally imply that the person repeatedly worked all night. That is, they were working every night over a non-specified series of nights.
1. Curiously, verse, which has a similar meaning to parecer, is used with a continuous form. So ya se estaba viendo cansada cuando... is grammatical.