In my schedule of preparations I experimented with different types of insect repellant. You might want to experiment with different mixtures before your run. There is stuff that works for mosquitos, but as long as you are moving, they can't bother you anyway.
Repellents are worthless against biting flies.
There isn't a mosquito around that can't keep up with you as you walk up a hill. By all means apply a good spray every few hours. There's no need to add to your problems by spending all night swatting bugs or continually forcing yourself to bend over to squash one on the back of your calf. If there is a good supply of ticks available, then be sure to spray your lower legs with a deet containing repellent. I donate enough blood to the briars, rocks and roots that enrichment of the insect life is an unjustifiable burden.