I have been training for my first 70.3, and therefore have upped my bike mileage. I have noticed that at about 40 miles i start having a burning sensation right on the ball of my foot, it can get pretty bad and probably slows me down a bit. Any advice?
TONS of advice - search here and every biking forum as this is a very common problem after x amount of miles (40 for you, 80 for another).
In general, move cleats back, loosen shoe, use a metatarsal pad/buttom, wiggle the toes in the shoes some, etc. But mostly, keep on biking/training and the “distance to burn” will increase (assuming no true pathology like a Morton’s neuroma developing, tarsal tunnel syndrome, etc.) 
shoulda done a search first! but thanks thats great advice