I took the Transpeak route to Manchester with a friend for a day out before.
But that was back in 2003.
A day return only costed £5 back then.
And the bus we rode was a Volvo B10M/Alexander Belfast.
Similar to the one below. http://www.flickr.com/photos/35777972@N02/3742528390/
I have to say we did not spend a lot of time in Manchester before we made our way back to Nottingham because the bus took more than 3 hours each way.
Peak District is quite popular among hikers & travellers across UK and there should be quite
a passenger load during weekends.
City buses on long intertown routes are no big deal even for large companies like Stagecoach,
which occasionally allocates Enviro 400s or even Volvo Olympians to Megabus services.