I’ve only ridden this with the family, but we’ve done it many times. Around the upper reservoirs from Fairholmes (the visitor centre signposted off the A57) is easy, we usually eat pies at the kiosk, and then ride partway around the lower reservoir, cutting back across the a57. It’s a little under 20 easy miles. Adding a bit more is going right around the lower reservoir, but this does mean a short ride back along the A57.
I’ve always wanted to do some of the more technical trails as well, I’ve walked most of them. You can ride up to Lockerbrook from the Bridge end carpark,on the road (west) side of the Derwent valley, lovely views down to the lake, and not too difficult looking a trail. On the east side you can ride up the field to Whinstone Tor – easy to miss the bridleway, it starts beside a gate at the top of a small climb – and down to the Ladybower Inn.
Lots of this can be muddy, and if you are aiming to go on a nice day in a weekend, parking can be difficult after 11 am