36 miles, or 18 miles each way is a pretty decent ride.

A few thoughts.

1. First of all, get a nice bike. I've had people at the bike co-op laugh at me for showing up on a well battered Colnago road bike, or using a titanium Litespeed as my rain bike. But there is a reason for that, as they're at about the 15 mile point in my ride if I'm just going straight there. I've read about the differences between skinny and fat tires... I still like the skinny tires. I now have a Cross bike, and it just doesn't feel the same.
   
2. Figure out what you need to pack back and forth, and what you don't. You didn't mention what you're carrying. For the most part, keep computers, cookware, etc at each destination. Just carry the bare necessities (such as groceries, flash drives, etc).
   
3. For the biking, you need to push yourself a bit, some of the time. Don't always stop at 18/36 miles. Perhaps go for a few 100 mile rides on the weekend. Play around with varying speed. You might not want to wipe yourself out on a Monday or Tuesday, but you could try using Friday as your "sprint and intervals" day. Occasionally snag your cargo trailer. Nothing beats climbing hills while dragging 500 pounds behind your bike.

As far as myself, I'm rapidly approaching the second half century of my life. I've always ridden a bike, but the number of miles varied. A couple of years ago, a ride from my house to Mom's house and back (about 20 miles each way) in a day would just wipe me out. Now, I'm pushing the speed and cargo capacity getting there, and the recovery time is much quicker. Still, a 100 or 150 mile ride is tough. I'm estimating 4000 to 5000 miles this year which will be the most in quite some time.