Best I can suggest which is a bit west of Birmingham rather than north, is the Six Shropshire Summits walk. It's about 36 miles.
I've only done about half of it, because I started it towards the end of the last foot and mouth fiasco. The first summit and about the first 5 miles was closed to walkers, and trail finding was made even more difficult by the local farmers' apparent decision to take advantage of the F&M regulations to allow footpaths to almost disappear. After struggling to find paths east of Church Stretton I came to a point where a stile was totally impassable so gave up and went to the nearest pub instead.
The route is a little difficult to get anyway. I used the Long Distance Walkers' Handbook which only tells you what the summits are, and leaves you to work out the route. I remember mostly enjoying it despite the frustration of trying to find public footpaths that had been allowed to decay.
S.