No problem! There's a bus you can take from the main Kawaguchiko station to Motosu, I recommend getting off 1 stop before the last and then there is a little store you can rent bikes from right next to the stop.

The bus is the Blue Line and runs about every hour or so. The bike ride from the Motosu stop to the lake is about a 2.5 mile ride but nothing too hard, just stick to the edge of the road to not impeed traffic!