The training plan that I just finished would mix it up. If you have time to ride on both weekend days, I would do speed intervals on Saturday, then do a long somewhat easier ride the next day. Do them in that order, so you have plenty of strength for the intervals.

I've always heard to add no more than 10% per week. So, start with your 50 mile rides, next week do 55, then 60.5, etc.