Many points already covered so will be brief!
Are you aiming to be fit or healthy? One of America's most noted runners (can't recall the name) died of a heart attack when he went running. He was fit but not healthy.
A heart rate monitor is essential . When I go to the gym I am amazed at how few people invest in this bit of kit. Also get the book "Heart Rate Monitor Book" by Sally Edwards which will give you all the info you need.
Always do a warm up and a cool down. I do 12-15 minutes at half maximum heart rate. Do not skimp on this since to do so renders the rest of the workout less effective.
As a fit (!) 52 year old who never did any exercise 10 years ago I can say that it has made a dramatic effect on every area of my life - the biggest benefit of physical exercise is the effect it has on one mentally.