I am on a beta blocker as well as a calcium channel blocker and ace inhibitor. I walk "briskly" EVERY day for an hour without any problems. I have had a couple heart attacks, have stents and suffer with vasospasms, and walking helps me with this. Begin your training at a slower pace and see how you do. If you haven't trained in 3 years give your body a chance to get into the routine again. Don't shock your body by training at a pace you haven't had any experience with for the last 3 years. You'll do fine. You didn't mention any other problems to consider, though. Are there any other concerns?
Take care, Ally
Shouldn't be a problem....
just searched "beta blocker and running marathon "....came up with this---
http://www.runnersworld.com/community/forums/index.jsp?plckForumPage=ForumDiscussion&plckDiscussionId=Cat%3ATrainingForum%3A658106477Discussion%3A277108028