I am presently a cyclist, doing about ~50 miles (~80 km) per week.
I am considering getting into duathlons, but I am concerned with injuring myself. I presently weigh 214 pounds (~97 kg), and I don't want to injure my knees, because that would set me back considerably (I'm currently 53 years old). I'd prefer to minimize this risk by putting off running until my knees have less to do.
How much should I lose before expanding into running?
I would suggest start running immediately. The sooner you start the sooner your joints and muscles will develop to deal with it. However, start small and cautious. Perhaps jog few hundred metres and see how your joints and muscles feel. This may seem like a pointless workout. To help with that perhaps you could go do this before going for your normal ride.
A common rule of thumb is to not increase running distance by more than 10% per week. I think this is too much for older runners.
I would also suggest looking up some advice on running form. Running correctly will reduce the force on your knees. For example landing with a foot too far forward is a common mistake for beginners.
