You say that your knees hurt. I often find that my new clients arrive with poor positioning on the bike. Commonly, the saddle is set too low which will tend to stress the knees, particularly if the rider is habitually using low cadence and big gears. Back pain is also often a result of poor positioning or over-big gearing, particularly when climbing.
I'm a bike fit fanatic. I'm absolutely convinced that getting --and keeping-- the bike fit correct is the first crucial step in the improvement process. As you become fitter your position on the bike is likely to change, so be prepared for adjustments in the future!
As Andy Pruitt says, position on the bike is a compromise between the human body, which is somewhat adaptable, and the bicycle, which is somewhat adjustable.
As we age (I'm 56) our joints and muscles become less resilient and less flexible and that adaptability decreases, making the adjustability piece even more important than before.
I recommend taking the bike to a licensed cycling coach, or to a good local bike shop, to get your positioning set up correctly.
Build a good base of mileage before getting into more stressful training like intervals. Enjoy the rides, and stay aerobic with a heart rate between 60% and 75% of your maximum on the flats, climbing seated at up to 85% of your maximum at a cadence of 65 rpm or more at first. A heart rate monitor is a really good investment.
Don't worry too much about speed when getting back into cycling after a long layoff. Use time on the bike to record your training.
Work on maintaining a good cadence (80 - 90 rpm) and occasionally spin up to 110 - 120 for a few minutes, using a very easy gear.
Good luck!