If your reactor and this routine has been loaded into memory, then just run an AutoLISP routine. The reactor will kick in and execute this routine. If you set a break point within it, then your code will stop when it hits the break point.
You can then show the Watch window with Ctrl-Shift-W (from memory it is listed in the debug menu). This will allow you to watch the values of variables you are watching. One of them probably should be the Last Value.
You can then step through the code using the F8 function key.