I would recommend wider tires. I had 37's on my bike and changed them to 42's - continental tour ride tires. If you are going a long ways you may run in to varying trail conditions - and surely some less-than-ideal spots.

With a load the 42's i have are manageable, but I have to watch the pressure. Ive hit mud and loose gravel with them and was sure glad I had those and not my 37's, but still no fun to ride thru regardless.

If you get new tires - get a more off-road tread. That will help on the gravel. That is one reason I went with the tour ride's - perfect for that type of trail.