I am pretty experienced backpacker, a novice cyclist--but theory is the same. Don't under-estimate the weather or over-estimate your ability to withstand it. Whatever you think the weather isn't going to be is likely what it will be. Take something beside a bag liner. You are minimally setting yourself up for an uncomfortable night.
I only carry about 30 lbs in my pack for 5 days, but I am damn sure it includes a solid sleep system.