I have bad public speaking anxiety. is it a good idea to go for a run beforehand to ease the stress? or is the added pressure on the heart a bad idea?

Great idea: You're on the right track- you want to find ways to reduce your stress. Many people use running for that since it releases natural endorphins - brain chemicals that help you to feel better. The fatigue is a natural way to help you stay calmer afterward as well.

Maybe: There are numerous techniques you can use to allay the fear of public speaking. Physical activity prior to speaking may be an effective way of coping. There are numerous agencies which help with media and public speaking training. These can really help with your problem.

If run helps it isOK: If running before helps you, than there is no reason why you can not do it every time you have to speak, but if it does not, you need to go for therapy as there may be underlying anxiety and therapy may help good luck.