Marathoner needs some advice

I have just started swimming and wanted to see if anyone here could give me some advice on a workout routine for adding some swimming workouts to my exercise routine. My main form of exercise for the past 8 or so years has been running. I have run 6 marathons, with a personal best of 3 hours flat. So I have decent endurance but don't have very good speed. I am basically just looking to add swimming to my workout routine to vary the routine and keep things interesting. I am not looking to become a particularly fast swimmer. I basically want to continue to work on my cardiovascular development and also work on toning my upper body...something that doesn't really happen when you run! I have been swimming for a little over a week now. I can swim *** stroke pretty much non-stop without getting winded. I am currently swimming a half-mile 5 times a week, with nearly all of my swimming being the *** stroke. If I go to the crawl, I tend to get winded pretty quickly. I associate crawl with speedwork like 400s or quarter miles that I do for my running. So with all that out of the way, my plan right now is to swim mostly *** stroke for half a mile, 5 times per week. I will throw in a little crawl to get my heart rate up and to add a little variety. Is this a reasonable workout? I'd like to work my way up to a mile for 5 times a week but I want to take my time and slowly build up my routine. Any thoughts, suggestions, pointers or recommendations would be appreciated!