Sorry,
laziness seems like a bad disease that clouds the brain.
Here is the solution:
(defun c:demo2 ()
(if (and (setq pline (car (entsel "\nSelect Polyline:")))
(eq (cdr (assoc 0 (entget pline))) "LWPOLYLINE")
(setq int (getdist "\nEnter Interval:"))
(setq in int)
)
(while (setq pt (vlax-curve-getPointAtDist pline int))
(progn
(command "_break" pline pt pt)
(command "_pedit" pline "_j" (entlast) "" "")
;--
(setq int (+ int in))
)
)
)
)