At your age it's probably wise to take at least one or two days off and then get back at it so you don't loose any fitness before your next ride.
But it depends on your fitness level and how hard you rode those two days. If you feel that your legs feel good just take one day off. But keep a check on your knees. For a new rider the muscles get in shape for riding much faster than the joints do so when you do a longer, harder ride your joints are not up to it yet. Often the way to deal with tender knees is to keep riding but at a lower speed and power output. My feeling is that your knees are tender because you may not be used to riding at that speed and your knees are not used to that kind of sustained power output. Try riding further at much slower speeds for a couple of days since the MS150 was a good workout you can back off for a few days to let your knees recover a bit.