For starters:- Nunburnholme, Bishop Wilton, Garrowby, Staxton, Flockton, Hanging Grimston, A few round Thixendale and Leavening...... As the others have mentioned look around a map, the one's you mention are not that steep compared to the above. In fact there's loads of them in the Wolds, which is why it is renowned for cycling in, and once you have done them all head North into the North York Moors
But where exactly is 20 miles from you?
All the hill above are 20 miles from Sledmere