Multi-modal commute

I have just started bike commuting again. I drive to the train station, take my bike on the train and then have a 4 mile commute on my bike once I get off the train. It's mostly downhill on the way into work, which is nice because I don't get too sweaty, but that means it's mostly uphill on the way home.

I'd love to bike the whole way, but it is 18 miles and 36 miles in a day would be too long for me at this point.