The reality is this is a tough commute. 18 miles + each way is a long commute under ideal circumstances (flat smooth pavement with no hills and great weather). Add the variables of gravel roads and some tough climbs and you've got a trek to and from work 5 days a week. Yes, of course, it can be done but it's a big challenge and it will occupy a ton of your time and energy. It's a big commitment and could be worth it if you want it for training purposes or you just want to dedicate that much of your time to this kind of riding. I had a similar commute a couple of summers ago and despite my best intentions couldn't manage it more than twice a week regularly. Weather and darkness were issues.
It's 3 hours of intense exercise per day and that means calories and fatigue. So you'll need to sleep a ton, eat a ton and if your work has much in the way physical or even mental energy demands it will be a challenge to keep that going on for an extended period.
Best of luck with it and keep us informed as to how it goes.