OK, 2 quick options for you (well they're longish rides- not that long, but for short days and not fast ground they're an ok length).
1. Alston- Nenthead on road. Over Flinty fell, turn left, drop down to Tyne, up towards confluence of the Tyne and Tees, climb up to Great Dun fell (this bit will be a nightmare if wet and is the worst bit of the ride- avoid if you don't like mud, river crossings etc (the track hangs over the river, or used to, it has now largely been eroded into the flow itself, leaving river or bog as options in some places). Off the tops on a variety of descents of your choosing, over the corpse road to Garrigill and road to Alston.
2. Alston- Garrigill on road. Up Cross Fell, down into Eden Valley. North through Melmerby, then east up Byways and bridleways towards Hartside summit. Down the road for a couple of k, Left on an obvious track on a corner for an off-road descent that brings you out near where you start.
2 is probably better for most people and wet conditions. 1 is good if you get a good cold spell, otherwise probably give it a miss. Whatever you do, avoid biking the blanket bog that is the bridleway marked on the OS that splits off the coach road and circuits around to join the pennine way to the south of CrossFell- non existent in places, always saturated if not under snow. It looks a tempting loop on the map, but if tempted its's always regretted. All the descents to the west are better than those to the east and north- much bigger drop, and steeper.