You can bind them off, then mattress stitch the two edges together (personally I’d avoid whip stitch, it doesn’t end up looking very tidy sometimes), or something else I’ve done is this:
Leave the stitches live on the needle and cut your yarn leaving however long a tail you think would be appropriate for seaming the entire edge. Thread the tail yarn through a tapestry needle. Pass the tapestry needle through the first stitch on the cast-on edge, then pass the tapestry needle through the first live stitch on your needle and slip the stitch off. Continue in this manner, alternating between the cast-on edge and the live stitches, snugging up the ‘seam’ as you go along to match the tension of the rest of the project as best you can. When you have sewn together all stitches, weave in the end as usual and cut your yarn.