I would start with walking/hiking during the cool time of the day. Can you walk for one hour five times per week? In the beginning, you don’t need to walk fast or far and just go along at an easy pace. If an hour is too long do less but do your best to get out five times per week. Do that for a few weeks and then increase your pace and walk faster. When you get to where you can walk fast for a whole hour and you are getting out 5 days a week, then you are ready to start cycling. If you can’t walk fast for an hour and do it frequently without feeling overly sore or tired, you are not ready for cycling.
Once you start cycling, start slow and go easy for short amounts of time. It’s better to go out more often on shorter, easier, rides than to do occasional, long, hard, fast rides. Build up your strength and endurance slowly to avoid injury. Try to ride for a minimum of one hour every other day-three times per week. Once you get to where you can do that without being overly sore or tired, then you are ready to ride farther, harder, longer, or more often.
Also, see your doctor to make sure you don’t have any health issue that would keep you from vigorous activity. Also, make sure your bike fits right. Listen to your body. If your heart is beating very hard, or you’re out of breath, or your sweating heavily, or something really hurts, it’s time to slow down or stop. Exercise should leave with a kind of soreness that actually feels good like you’re feeding your muscles. It shouldn’t require ice packs and pain meds.