The jump from 30 to 50 miles is quite large this early in the season especially if you significantly increased the amount of climbing relative to the 30 mile rides. This aching is to be expected. Give it some rest or a recovery ride and you should be good in 24-48 hours.

Additionally you are not as limber early in the season as you are later if you have not been training in the off-season so it may be that your ligament was stretched a bit too much. Some people lower their saddle 1-3mm early in the season to compensate for this. I prefer to ease into the season by increasing my riding distance week on week by 15-20%.