Swimming as cross training

After reading some of the other postings I see I am out of my league here. I have never been a competitive swimmer, though swimming has been a lifetime pleasure. I am a runner. Not elite, but I enjoy 10ks and half marathons. I started lap swimming again 2 weeks ago for cross training purposes. It feels great. I know that in running, a rule of thumb is to not increase your distance by more than 10% per week. Is there something like that in swimming? I swam 1,000 yards (meters?) today in 35 minutes - a piker to all of you, I know - but if I am swimming 2x per week, does anyone have any general tips for me as I build up to an hour of swimming each time and to build my speed up some? Laura