There are two main routes into Boso Peninsula from Tokyo:
To access the southern half of the peninsula, it is convenient to take the ferry from Kurihama ferry terminal on the east coast of Miura Peninsula. The ferry takes 40 minutes across Tokyo Bay to Kanaya on the west coast of Boso. There are direct trains from Shinagawa to both Kurihama and Keikyu-Kurihama on the JR Yokosuka and Keikyu Lines. It is then a short ride to the ferry terminal where you can ride straight on to the ferry. As soon as you leave Kanaya ferry terminal, you have good riding. On the ferry, there is a shop selling coffee and snacks, and they even provide slippers so cyclists can take off their cycle shoes.
Apart from the regular trains running east out of Tokyo to Boso Peninsula, there are a number special "bicycle trains" which allow you to take your bike without putting it in a rinko bag. The B. B. Base (Boso Bicycle Base) trains run from Ryukyu Station to various destinations in Boso. There are various rules and restrictions, so check the website carefully.
Kanaya - Futtsu - Central Boso - Shirahama - Tateyama - Kanaya (114 km loop, road)
Access: Starts & finishes at Kanaya Ferry Terminal; Kanaya Station nearby
Starting from the Kanaya Ferry Terminal, this route heads north along the coast for 10 km and then cuts inland from Futtsu. It then follows Routes 88 and 86 down the middle of Boso Peninsula to Shirahama on the south coast. The roads are quiet and undulate through rice fields and forests. From Shirahama, you follow the coast all the way back to Kanaya. After Tateyama, the road is a bit busier, but not too bad.
Kanaya - Futtsu - Shirasuka Kaigan - Shirahama - Tateyama - Kanaya (126 km loop, road)
Access: Starts & finishes at Kanaya Ferry Terminal; Kanaya Station nearby
This route starts the same way as Boso Route 1, but half way down Boso Peninsula, it cuts across to the south-east coast at Shirasuka Kaigan and then follows the coast all the way back to Kanaya.
Yotsukaido - Itabu - Yotsukaido (118 km loop, road)
Access: Starts & finishes at Yotsukaido Station (less than one hour by train from Tokyo Station on the Sobu Line (Rapid), no changes)
This route starts at Yotsukaido and then winds south to Itabu in central Boso Peninsula along quiet, undulating roads. There is a pizza restaurant with indoor and outdoor seating, Pizzeria Bosso, next to Takataki reservoir, just past the halfway point.