Setup the bike properly and spin a lower gear at a higher cadence and you can avoid most knee injuries.
30 miles is no joke for someone who hasn't been on a bike in 15 years. I did 35 miles after not biking for a couple years and did some nice tendon damage. I felt fine on the ride (albeit tired), felt fine after the ride. Knee started to ache later that night, popped some ibuprofen and went to bed. Knee felt really sore for a few days, went to try riding again a week later and it was killing me. Took months to heal.
Start at 5-10 miles and work your way up. Your muscles might be able to handle it, but your knee tendons need to develop before you go on longer rides.