Three stops on a local JR line from Hamamatsu is JR Bentenjima Station.
It is a fairly industrial area, heavily concreted by the Ministry of Construction. Just across a large road though is the Pacific Ocean.
Close to the beach is a decent rental shop where one can rent a beater of a bike for just 500 yen/day. After about 10 minutes on said beater, the scenery improves. After about 20 minutes, you are cruising on a bike lane that skirts the edge of Lake Hamana, or "Hamanako" in Japanese.
Hamanako is a large lake on the Pacific Coast in Shizuoka Prefecture. It was originally a fresh water lake that was not connected to the Pacific Ocean.
However, in 1498 a large earthquake opened a pathway from the ocean to the lake, with the result that the lake today is salt water.
The full bike trail loop is 82 km, but of course you can choose to do less than that.
From Bentenjima Station, go out the exit on the side of the Pacific Ocean. Walk towards the ocean. There is a rental shop in the plaza in front of the beach called Yurin.
To rent a bike, you will need to fill out a form with your name, address, and telephone number. The rental fee is 500 yen plus a deposit of 1000 yen, which is refunded after you return the bike.
This is one of several "terminals," where rental bikes can be had.
Tel: 053 596 4187
Map in Japanese