Since it's a random access iterator, you can take the difference of the two to get the distance between them:
i = myvec.end() - it;
However, this requires a random access iterator. If you decide to change the container to a list, for example, the trick no longer works. The standard library supports a distance function in the <iterator> header that will do the right thing regardless of iterator type:
i = distance(it, myvec.end());
If the iterator type is random access, the difference trick will be used. Otherwise it falls back onto the less efficient looping methods.