C++ while loop with initial check?

As a programmer I faced the following problem many times and never was able to solve it, suppose I have:

bool is_succeeded;

while ((is_succeeded=my_fun()))
{
    if (!is_succeeded)
    {
         //really long code
    }
}

This will bypass the initial check (if my_func failed at first time and won't call the //really long code part).

A possible solution would be to add another condition before or after the while loop for the initial check, but that will cause a lot of duplicated code, how can I solve this problem?

For example I don't want the following code:

bool is_succeeded=my_fun();
if (!is_succeeded)
{
    //really long code
}
while (is_succeeded)
{
    is_succeeded=my_fun();
    if (!is_succeeded)
    {
         //really long code
    }
}

Why?

Because I am writing //really long code part twice which is a duplication of code.

maybe do while can help here ?

Please Note: I know I can move //really long code to a function and call it but for my own reasons (hard to explain) I can't always do that.