I have been skating along the East Lancs Cycle Path over the past couple of weeks training. The path is smooth, has few side entrances and you can do out and backs of around 20 miles from Walkden (nearest station). The best place is from the Worsley / Boothstown turn-off towards St Helens. Just be friendly towards pedestrians and cyclists as they are not used to seeing skaters on the path. Though because of the lack of bus or train routes, the majority of people are leisure / commuter cyclists.
Recommend using a brake as there are a couple of flyover bridges and footpath side entrances. The path runs smmoth until Knowsley, but then changes to the 1970s pavement slab surface towards Everton/Liverpool.
I tend to get down there around 1745 and skate until 2000 as the train back to Walkden is at 2049.
For proper speedskate training, I would recommend Preston Arena. The cost is £4 for a day pass, but you can skate on the track on a fast, flat surface. The only negative is getting there as it is about an hour walk from the train station or a bus ride with a walk along the canal. Taxi is probably the best option. Will cost a bit to get there, but worth the effort.
Alderley Edge is probably the best place around the NW though. Easy to get to, shops nearby for refuelling before heading back out again and very quiet on the route. The only negative is that lack of trains on Sundays.
These are the places I have been skating on over the past year in the NW. It is just a shame that some of the old railway lines or canal paths are covered in gravel or grass and not tarmac e.g. Leigh to Eccles.