Conversion of integer pointer to integer
Apparently you confuse the pointer with the content of the pointer.
As an analogy to the real world, you could say that, with me pointing at a bird, you want to convert my index finger to a bird. But there is no relation between the type 'bird' and 'finger'.
Transferring that analogy to your program: you are converting the object pointing to your int to an int itself. Since a C pointer is implemented as 'the number of a memory cell', and since there are lots of memory cells available, it's obvious that (int)p will result in a very big number.
Casting is a nasty thing. It's a coincidence that pointers are quite analogous to integers. If they were implemented as "the nth address of the mth memory bank", you wouldn't be asking this question because there wouldn't have been an obvious relation, and you wouldn't have been able to do this cast.
135680008 is the address in decimal (it would be 0x8165008 in hex) to which p is pointing: the address of the memory area allocated with malloc.