As far as I know, the best riding is all N of Derby, but it's worth riding round to.
If you get the train / drive up the A6 from Derby up to Belper, Cromford, Matlock, there is absolutely tons of great riding that way, it's the edge of the Peak District. I did a 40 miler recently from Matlock back home to Belper without duplicating any trails, and I can easily think of another 40 miles you could add on to that, there is that much good riding, and it's easily accessible by train too which is a bonus.
You can go on further into the centre of the Peak District (Bakewell, Curbar etc.) , although that is a bit of a mission from Matlock on the bike if you don't have access to a car.
My condolences on picking the wrong end of the Brian Clough Way.
Not for bike riding. Nottingham = an okay place, but you have to drive miles or catch the train to ride your bike anywhere decent, otherwise you're confined to Bestwood / Shipley / Bramcote etc. which are nice enough local riding, but not quite the same as what you can ride to if you start in Derby and go north.
Joe