You could easily do 15 easy miles along the carriage road of the preserve and mountain house. I'd suggest the following trail sequence. You can check the NY-NJ Trail Conf. maps for distance, but this is pretty flat:
From the Trapps:
Overcliff to Rhododendron Bridge
Oakwood to Glory Hill
Continue on Oakwood and onto Forest Drive
Then Lenape Lane takes you over Mountain Rest Road and out to the Spring Farm Area
You can go out to as far as Table Rocks, but then you'd have to retrace your steps.
For the way back, coming back from Spring Farm, you can walk the other side of the ridge.
You can grab all the trail names from the aforementioned map.
You can pretty much do the whole ridge hardly any elevation change. And it's a beautiful walk to boot. I would come back on the Old Minnewaska trail and wind up at the Coxing Kill for a swim. That trail is a long slow descent, so I don't know if it fits your criteria.
What are you training for??