SQL & PL/SQL :: Recursive Query - Each Field Starts Where Previous Ends

I have a table:

create table FIELDS
(
FIELD_NAME VARCHAR2(30) not null,
PRG_FIELD NUMBER not null,
LENGTH NUMBER
);

with

INSERT INTO FIELDS VALUES('FIELD1', 1, 3);
INSERT INTO FIELDS VALUES('FIELD2', 2, 3);
INSERT INTO FIELDS VALUES('FIELD3', 3, 4);
INSERT INTO FIELDS VALUES('FIELD4', 4, 2);
INSERT INTO FIELDS VALUES('FIELD5', 5, 1);

I need to insert in a table:

create table STUFF
(
FIELD_NAME VARCHAR2(30) not null,
FSTART NUMBER not null,
LENGTH NUMBER
);

And the output I want is:

INSERT INTO STUFF VALUES('FIELD1',0,3);
INSERT INTO STUFF VALUES('FIELD2',3,3);
INSERT INTO STUFF VALUES('FIELD3',6,4);
INSERT INTO STUFF VALUES('FIELD4',10,2);
INSERT INTO STUFF VALUES('FIELD5',12,1);

So each field starts where the previous (ordered by PRG_FIELD asc) ends.

I think the query should use both lag and connect by but I haven't had any luck writing it. The problem is that all the examples I've seen around, using connect by prior, utilize 2 fields with different names, es connect by prior emp_id = mgr_id. Instead I should do something like connect by prior prg_field = prg_field-1 but that doesn't seem to work.

PS: I don't necessarily need to do this, I have a guy manually writing the inserts, this is just an exercise I would like to figure out