If you have lower back pain while running you need better shoes to absorb the road shock to ankles, knees and hips. Don't worry about how fast you can run a mile. You are not racing; you are training.

Stretching before and after exercise is good because it warms and loosens the muscles and reduces the chance for pulled muscle injuries.

What helps the heart, lungs etc is the amount of continuous time you spend exercising --about 30 minutes at your target heart rate.

Combining biking and running can only do you good.