Fellow Boltonian here.
The Middlebrook trail runs from the 3 tower flats in Deane to the beehive roundabout in Horwich. Along the railway line. It can be a bit bumpy in places but can be done with a road bike easily.
If you wanted to join a social group on rides,
https://www.bolton.gov.uk/sport-bolton-0/walking-cycling/1
Scroll to where it says Ride this Way.
I used to cycle with them myself and are a friendly bunch.
Sadly, nice slow cycle clubs in Bolton don't exist. They're all for racers and speed merchants.
If that's your thing, there's Bolton Clarion.
A few others too but I can't remember them off heart.
Cycle routes are generally a joke in Bolton so I can't really advise on any. To get any half decent cycling done, you either need to go north towards Rivington where the moor roads can be a nice break from the urban hell that Bolton has become, south in Cheshire where its flat ish and leafy, west towards and beyond Chorley or go east past Bury for some hilly Yorkshireness.