WIll be finishing up my first full 5-day week of bike commuting - 18 miles (flat) out and 18 miles back.
Takes me an hour each way, and that's hammering most of the way at 22-24mph whenever I'm moving. The inevitable lights and stop signs pretty much kill your average speed.
I'm doing it with a BanjoBros standard size backpack, which holds as much as a large grocery bag. I thought it would be too bulky but it's great. I loaded more and more into it until yesterday I had a pretty hefty bag and it still was no problem. I actually think it gives less wind resistance than dual panniers on a rack, just from my experience.
Fatigue definitely wasn't ever a problem, but I come from a triathlon and marathon background so 2 hrs a day of bike commuting, even at speed, is not difficult compared to running every day for about that much.
Keys to being happy at work - shower BEFORE getting on the bike, use clean cycling clothes, and have some cool-off time before changing into work clothes. I'm drenched in sweat by time I arrive at work, but I'm good to go with minimal fuss after I'm cooled off.
I still haven't had to deal with darkness yet, but I'll be getting a MS for that soon. Already running a Mars3.0 on back, which seems plenty bright.