cumulatively calculate the distance along a line

I am using Postgis to: - calculate the cumulative distance in metres along the line - store the cumulative distance in "m" I have a table with points on this line and each row holds lat and lon coordinates. The "m" value is 0 for all rows. The following code nicely gets me the distance between the first point and the next.

SELECT a.geom
FLOOR(
ST_Distance(ST_Transform(a.geom, 3857), ST_Transform(b.geom, 3857)) * cosd(42.3521))
AS dist
FROM line a
LEFT JOIN line b
ON a.id = b.id + 1

however, it does not add up to a total nor does it update the "m" value yet. I tried a set of permutations of the following, but none of them store a cumulative sum of the previous row's value for "m" plus the calculation of the distance between this point and the previous.

WITH next AS (
	SELECT
	ST_Distance(ST_Transform(a.geom, 3857), ST_Transform(b.geom, 3857)) * cosd(42.3521)
	AS dist,
	a.id AS rowid,
	FROM line a
	JOIN line b
	ON a.id = b.id + 1
)
UPDATE line a
SET m = FLOOR(next.m + next.dist)
FROM next
WHERE a.id = next.rowid
RETURNING a.m, next.dist;

or

update line a
    set m = FLOOR(a.m + prev.dist) 
    from (select
		  	l.*,
		  	ST_Distance(ST_Transform(
				lag(geom) over (order by l.id asc)
			, 3857), ST_Transform(geom, 3857)) * cosd(42.3521) as dist
			from line l
		  ) AS prev
	where prev.id = a.id - 1
	returning prev.m, prev.dist, a.m

Any ideas?