Hello Nigel
Take it from an old guy,(48) if thinking about feet and shoes is the ONLY way you can get excited, then yes you my have a problem. If you just think about it every now and then, dont worry. When I was your age I thought about all kinds of stuff. Some people like big breast, some like long legs, long hair,short, tall, big, small..etc....
When mastubating, its not uncommon for all sorts of thoughts to go through your head. I still think about all sorts of things. I think you just happen to be turned on by womens feet. Thats ok because women like that. I dont think you have ANYTHING to worry about. Your normal.....
Tex