I can't understand why people use the gym when you can use a bicycle, run, swim, mow the lawn and do gardening. Cycling is great fun, always better if you can get others to go with you. As the others said take your time, use the gears, don't lose enthusiasm and make sure your tire pressures are correct otherwise you'll be knackered in no time. Also worth thinking about tubeless/slime and/or any other puncture proofing solution. If you're only going on the road make sure you have a racer rather than a mountain bike, alternatively a hybrid is a good compromise.
Opinions are like A**holes, Everyone has one. Some people seem to have more than one though which is a bit odd.