Travel in space AND time with a visit to the Isle of Wight!
Riding around the island on the well-marked on-road cycle route is 100km, with an extra 10km if you ride to the Needles. For hills, it's constantly lumpy rather than alpine. I can't vouch for off road, but the recent extensive resurfacing of the island's roads means they're better for riding than anything on the mainland. There's lots of coast to enjoy. The north-facing coasts are touristy and sometimes a bit shabby, but the south western edge has spectacular chalk cliffs, quiet beaches and is largely unspoiled.
Downsides. It can get busy in high summer. The ferries across are expensive. The local driving style is a little retro and some islanders can carry a bit of a chip on their shoulders. If you're old enough to remember the 70s, you'll get the vibe.