Being 6'5 and still 260 (down from 270) I understand your pain. At on point in life I was 300pnds. Now though I am far leaner with huge legs. But anyways to my recommendations.

1 - listen to your body. HOWEVER because you haven't ridden in years you could take this too far.Aches in the muscle are usually a good thing aches in joints (especially knees) aren't good and usually require some rest and ice AS WELL as movement. Meaning stretch the joints.

2 - Stretch. You are big. Bulky. Your muscles probably aren't used to exercise. Stretch them and warm them up. This will save much pain later.

3 - build muscle by working the sub groups that aren't as prominent in biking. Shoulder, arms (biceps and triceps), back (ESPECIALLY if trying mountain biking). But don't overdue it

4 - Enjoy riding. As long as you enjoy it keep it up. If you find yourself resenting the activity take a week off.

Other than those tips this is a great sport in all aspects. I personally mountain bike and prefer trials, dirt jumping, single track over road. But all are great for fitness as long as you moderate your activity and learn how to read your body.