We have stayed at the Rampsbeck Country House Hotel. It's right on the Ullswater on the A592 south of Penrith Cumbria.
The surrounding hills are honeycombed with paths, byways, and old droves suitable for hiking and/or mountain biking. There's also a couple of farms offering horseback riding. There's a boat service that can take you down to the Trust or Information Centre in the southern part of the lake. Hence, a good place to set your 'base of operations'.
There's a sports store across the street and a small grocery store where you can stock up.
The hotel itself is pricey. In fact very pricey. But great accommodation and the Sunday roast is packed.
Transportation: the hotel is a 3 minute walk from the Watermillock Train Station and trains run daily from Penrith. By car, be prepared for arduous driving once you leave the A66, also mobile dips on the A592 that disable satnavs.
I have no connection to the hotel other than previous occasions as a customer. Trip Advisor reviews at http://www.tripadvisor.co.uk/Hotel_Review-g1096480-d191540-Reviews-Rampsbeck_Country_House_Hotel-Watermillock_Lake_District_Cumbria_England.html
Walking routes, hiking paths, and hiking information for Ullswater is at Walking and hiking around Ullswater
(Images from Cumbria Tourism, with permission from Shaun Walby, also with permission from helvellyn)