Problems with rising sponge in larger tin
I see nothing wrong here. The two 7 inch tins have a total area of almost 310 square inches, while one 8 inch tin has a total area of 200 square inches. So your batter is 1.5 times thicker in the one tin. 

Baking time does not increase linearly with thickness, so a doubled time is not out of the ordinary. Time is not a prescription in baking anyway, you have to bake it until done and not until the timer goes off. 

The "hard on the edges, gooey in the middle" is a classic sign of too high oven temperature. Reduce the temperature to bake the thick cake. 

Alternatively, make only 2/3 of the batter, or find a 10 inch tin (314 square inches area).