After being a competitive runner for 20 years, I am making a serious attempt at cycling this season.
When I was running only I averaged about 70 miles a week, often much more than that. Now that I'm mainly cycling I only run about 20-25 miles a week (usually 3-4 runs of 4 to 7 miles). I don't think that short easy runs of 30 minutes or so will hinder cycling, and if you only have 30 minutes to workout you can get more of a workout from a run, but I try to bike instead of run whenever I can.
Also, if possible try to do your running on soft surfaces, such as trails. Running a lot on pavement can really tear up your legs, and hinder your ability to build up new cycling muscles.