Give yourself some time between rides. Hydrate. Drink lots of water. I've found that if I drink plenty of water that I don't have sore muscles or pain at all the next day. And I mean alot. Do some mild stretches before riding to warm up. Back pain could be do to riding position. Change your hand positions or fiddle with seat height, bar height, etc. That often helps solve mysterious pain.

Keep on trying. I've lost lots of weight through cycling. I'm quite proud of how if you change your outlook, you can change your life. Don't give up.