Which section are you looking to do ? How far ? Do you want to do a "there and back" ride, or find a circular route, or perhaps get a train back ?
I'm planning to do the whole route west to east at end of August (if my knee recovers in time) and I've already done Leeds to Chesterfield. The latter is about 65 miles, some of which can be fairly rough trails.
Dunford Bridge is a good starting point as Cubist says. Going east from there it's all downhill, well basically all the way to the coast at Hornsea! Couple of minor inclines maybe. But you can certainly do a good long ride before you turn back - the return journey would be uphill though. It's only a slight incline but at Dunford Bridge you're very close to the highest point on the route. Also the track to Penistone can be a bit muddy and rough. DEFINITELY not for road bikes.
Alternatively, the path between Sheffield and Penistone is very well surfaced, flat and with good car parks at either end. How about starting at Meadowhall, heading up towards Penistone and turning round when you've had enough. Starting at Penistone would work well too.
Old Moor RSPB (SE of Barnsley) has good car parking, a great cafe and of course other things to do than biking. From there the paths are good in either direction. You can head E towards Doncaster, or W towards Penistone - you can also branch off North or South. Lots of choices.
