If you have not checked your patch kit in the past year, you may want to check and make sure the glue hasn't dried up on you. Try to take a couple of extra spokes with you. Luckily I only had to carry my bike about a mile and a half. I rode over a stick that curled back up and through my wheel, breaking several spokes. I now carry about 6 extra spokes in my seat tube.
Wider Tires are better.
Not necessarily knobby tires, just not really skinny racing tires. There are quite a few sections that racing tires would be fine, but then there are some sections that they would be inappropriate. Try not to plan too tight of a time frame for your trip. The towpath is constantly changing. The Park Service is trying to fix bad sections, and mother nature trys to undo some other sections.