Tendons and joints are the easiest things to injure. Muscles can take a lot of punishment. So can the cardio-vascular system--the heart is a muscle after all.
But, you probably need to give your tendons more time to get back into shape after 4 years off. They will need much more time than anything else. I would think a year minimum for the kind of riding you are talking about (but, if you are young enough, and have done other sports in the meantime, you might be able to handle it).
More importantly, tendons also take much longer to heal than muscles. With tendon injuries you generally shouldn't try to "ride through" the pain. I lost 2 months this summer due to an Achilles tendon injury. Trust me, you don't want this.
To go easier on the tendons and joints, the best thing you can do is to use a high cadence (over 90 rpms). That will shift the work to your heart and away from the legs. Hence with each pedal stroke, you'll be putting less strain on the tendons and joints.
Cheers,
Jamie