There’s a bridleway runs around the back of the lake. Good XC stuff, pretty flat, so no huge climbs, but very technical in places. I’ve only done it once, but I’m pretty sure that was under 3 hours to do a full lap of the lake (including the initial ride up the road).
Then there’s Helvelyn too. Sounds like an all day epic, but I did both in a day – and it wasn’t that long a day. If I remember right, I think it took me about 3 hours to do Helvelyn. Obviously loads of climbing, but it’s like a 10 mile round trip or something. I went up Keppel Cove (the ‘easy’, mostly rideable, way up), and back down sticks pass, which is steep and technical in places.