Agree with Ben. A good headtorch is a worthwhile purchase.
As GKD said, start learning what your effort levels are. Race pace runs on the course a great for this, or similar terrain if you are not able to get to the course before the race. Any recce opportunities are worth while too!
You can obviously use the watch early on when everything feels real easy to keep yourself on track but I really wouldn't waste the time and effort trying to charge it on the go. Running is very simple, if you keep it that way you will have a lot of fun over 100 miles