Don't be afraid of working with a circular needle. It is not used only for knitting 'in the round', it can be used for knitting flat as well. Circular needles are often called for when making things like afghans because they can hold a lot of stitches, a lot more than straight needles can. Also, the weight of a blanket in progress is a lot more manageable on circular needles. The weight of it is kind of held in your lap rather than the whole thing having to be supported by your wrists on a straight needle.
So, if you have straight needles long enough to hold the stitches then by all means, but you'd likely be a lot more comfortable working with a circular needle in the long run.