Swimming AND Running!!

Hi!! I'm new to swimming and I must say I find the contributions on this forum really good so I am hoping for some sound advice. I am a member of a gym with an indoor 25m pool and am struggling to improve the number of lengths I am currently doing which is only about 30 in 25 minutes. I acknowledge that my stroke is probably lousy and as a result I am taking lessons from professionals at our national 50m olympic pool (I am lucky to have such a facility nearby). I have been a good runner for a number of years and I was wondering whether it is a good idea to do some running in tandem or on days when I am too sore to swim. I did read that running and swimming build different types of muscle which are polar to one another, is this true? I want to join a local masters team here in Ireland but don't know how good I need to be before I approach the team!!! I would appreciate any advice