Leave Kirkby Lonsdale via Stone Rigg Outrake through Dent and Cowgill to Hawes, then visit Hardraw Force before heading over Butter Tubs pass to Muker then on to Thwaite. Around 35 miles so you need a few stops and/or browse an OS map for some diversions.
Do you want a 6 hour round trip back to Kirby Lonsdale ? If so the above route continues with another big climb out of Swaledale over to Kirby Stephen and then a bit more more climbing passed the station before a long, long fast descent back to KL.
If you think that is a lot of climbing in six hours, the route direct from KL direct to Hawes via Garsdale Head avoids the climb out of Dentdale up to Ribblehead, but does swap the quiet lanes via Dent for a slightly busier (motorbikes!) more main(ish) road.