Have you tried the Jarrold Pathfinder guides (the "Green Books")? There are two covering Snowdonia - "North Wales, Snowdonia and Offa's Dyke" and "Snowdonia, Anglesey and the Lleyn Peninsula Walks". They cover every level from three mile strolls up to easy hillwalking, and are probably at their best in the sort of range you're looking for (as opposed to the hillwalks, which tend to be popular hills by the tourist routes, along with dire warnings about not doing them in bad weather).
Another one to try is the Collins Rambler's Guide - North Wales by Richard Sale (stop tittering at the back - bobble hats and red socks are optional these days ;-) It's mostly pitched at decent hillwalking, and goes up as high the Snowdon Horseshoe, so you could also view it as an investment for when the knee's better, but it includes some lower level walks as well. Flicking through it, some of the easier ones have stats like: 6 miles, 275 m ascent; 8 miles, 230 m; 8 miles, 35m; 5 miles, 200m.
Both of those series are pretty widely available, so you should be able to flick through them in any decent bookshop before you buy and see how inspiring you find them.
